Physical forms of MXene materials exhibiting novel electrical and optical characteristics

ABSTRACT

The present invention(s) is directed to novel conductive Mn+1Xn(Ts) compositions exhibiting high volumetric capacitances, and methods of making the same. The present invention(s) is also directed to novel conductive Mn+1Xn(Ts) compositions, methods of preparing transparent conductors using these materials, and products derived from these methods.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/513,740 (filed Mar. 23, 2017); which is a national stage applicationof International Patent Application No. PCT/US2015/051588 (filed Sep.23, 2015); which claims priority to U.S. patent application No.62/055,155 (filed Sep. 25, 2014) and U.S. patent application No.62/214,380 (filed Sep. 4, 2015). All of the foregoing applications areincorporated herein by reference in their entireties for any and allpurposes.

GOVERNMENT RIGHTS

This invention was made with government support under Contract No.1310245 awarded by the National Science Foundation. The Government hascertain rights in the invention.

TECHNICAL FIELD

The present invention(s) relates to novel physical forms of conductivetwo-dimensional M_(n+1)X_(n)(T_(s)) MXene compositions, and methods ofmaking the same.

BACKGROUND

The search for new electrically active materials is driven from a widerange of potential applications.

Safe and powerful energy storage devices are becoming ever increasinglyimportant. Charging times of seconds to minutes, with power densitiesexceeding those of batteries can be provided by electrochemicalcapacitors, in particular, pseudocapacitors. Recent research has focusedprimarily on improving gravimetric performance of electrodes, but forportable electronics and automobiles, volume is at a premium.

In the search for new electrode materials, two-dimensional, 2D, solidsare of particular interest due to their large areas ofelectrochemically-active surfaces. For example, the use of activatedgraphene electrodes versus conventional porous carbons can result incapacitances of 200-350 F/cm³ (compared to 60-100 F/cm³ for activatedcarbons). Yet graphene is limited to the chemistry of carbon, does nottap into metal redox reactions as in RuO₂, and its conductivity issubstantially decreased by redox-active functional groups.

The best volumetric capacitances of carbon-based electrodes are in the300 F/cm³ range. Hydrated ruthenium oxide, RuO₂, utilizes highlyreversible redox reactions to reach capacitances in the 1000-1500 F/cm³range combined with great cyclability, but only for thin films.

It is an object of the present invention to address at least some ofthese challenges, or to provide a useful alternative.

In other applications, conductors that are extremely thin can betransparent and the fabrication of transparent conductors (TCs), acritical element of touchscreen electronics and solar cells, is abillion dollar per year industry. Transparent conducting films (TCFs)are optically transparent and electrically conductive in thin layers.They are an important component of a number of electronic devicesincluding liquid-crystal displays, light-emitting diodes, touchscreensand photovoltaics.

Transparent conducting films can be used as windows through which lightpasses to access a photoactive material beneath (e.g., a photovoltaic,where carrier generation occurs), as an ohmic contact for carriertransport out of the photoactive material, and can also act astransparent carrier for surface mount devices used between laminatedglass or light transmissive composites.

Indium-tin-oxide (ITO) is the most widely used transparent conductorsbut is limited by the high cost of both the raw materials, such asindium, and the fabrication technique. Fluorine doped tin oxide (FTO),and doped zinc oxide have also been used for such applications. Morerecently, films using materials such as silver nanowires or carbonnanotube networks or graphene have been used as an alternative to ITO.Such materials are particularly useful owing to their transparency toinfrared light. These all represent ‘bottom-up’ nanomaterials, requiringexpensive synthetic procedures to make the starting material. Thehighest conductivity reported for solution processed graphene is 200S/cm and this material has not been shown to demonstrate transparency inany applications.

It is an object of the present invention to address at least some ofthese challenges, or to provide a useful alternative.

SUMMARY

Herein are described a new method to produce two-dimensionalM_(n+1)X_(n)(T_(s)) compositions, in some cases exemplified byTi₃C₂(T_(s)) (where T_(s) refers to surface terminations). These methodscomprise etching the aluminum from Ti₃AlC₂ using fluoridic acids, forexample aqueous HF derived from mixtures of alkali fluoride salts andmineral acids. The resulting hydrophilic, water-swelling material can beshaped like clay and dried into a highly conductive solid or rolled intofilms tens of microns thick. Additive-free and binder-free films,produced by rolling, demonstrated volumetric capacitance on the order of1000 F/cm³ (at 2 mV/s and 5 micron thickness) with excellent cyclabilityand rate performances. This enhancement represents an almost three-foldincrease in capacitance over previous reports.

The present invention will be generally described with reference toTi₃C₂ as an example, although it will be appreciated that the scope ofthe invention is not limited to this particular example but encompassesall compounds of the general M_(n+1)X_(n)(T_(s))formula, their solidsolutions and composites based upon them.

In some embodiments, the reaction of titanium aluminum carbide withfluoridic acids, such as derived from hydrochloric acid and lithiumfluoride, after equilibration with water to adjust pH, a material can beproduced which dries into a solid mass with high conductivity. Analogousto clay, upon addition of water, the solid mass is rehydrated; it can bemolded into a desired shape and dried again to the starting consistencyand properties; this process is repeatable. Further, the material can berolled under pressure to yield free-standing flexible films that arevery conductive; rolled thinly enough, these films can becometransparent (visible light can pass through without scattering).Dispersed in water, the material can also be drop casted to yield thin,conductive, highly transparent films on various substrates (plastic,glass, etc). Like certain clays, the material shows variable swellingupon water uptake. Further, processing by rolling yielded electrodes forsupercapacitors showing volumetric capacitance on the order of 1000F/cm³. Higher capacitances are believed possible with optimization.

Generally, clays are non-conductive and typically require conductiveadditives to show conductivity. This new ‘clay’ is both hydrophilic andshows high conductivity without additives upon drying. Due to theseproperties, the hydrated material can be processed by rolling into filmswith highly controllable thicknesses (from a few microns to more than100 microns), with virtually no lateral size limitations. These filmshave recently been tested in electrochemical capacitors(supercapacitors), demonstrating exceptional volumetric capacitance onthe order of 1000 F/cm³ (at 2 mV/s in sulfuric acid); further, the costof materials is low since the active material's composition is of carbonand titanium. Such ease of processing, high performance, and relativelylow material cost is a breakthrough for supercapacitor electrodes.

This disclosure also provides improved methods for intercalating alkalimetal and alkaline earth metal ions within the MXene frameworks,providing new methods for tuning the electrical and opticalcharacteristics of these materials.

Also disclose herein are new methods of producing Angstroms- ornanometer-thick films comprising M_(n+1)X_(n)(T_(s)) compositions, againexemplified by from MXene-phase titanium carbide nano platelets such asthose derived from Ti₃C₂(T_(s)) (where T_(s) refers to surfaceterminations). Certain of these films are optically transparent owing totheir composition and thicknesses. In some embodiments, the MXenenanoplatelets, dispersed in a solvent, are cast on to an arbitrarysubstrate, for example by spin coating or dip coating, and the solventevaporated leaving a thin film composed of MXene flakes that areAngstroms or nanometer thick. The films are highly conductive,exhibiting an intrinsic conductivity of at least 1000 S/cm (in somecases over areas as large as one square inch), while attractively lowabsorption coefficient on the order of 10⁵ cm⁻¹.

In many cases, the present invention will be generally described withreference to Ti₃C₂ as an example, although it will be appreciated thatthe scope of the invention is not limited to this particular example butencompasses all compounds of the general M_(n+1)X_(n)(T_(s)) formula,their solid solutions and composites based upon them.

In some embodiments, a method comprises (a) applying a MXene dispersiononto a substrate surface, said MXene dispersion comprising or(consisting essentially of) at least one type of MXene plateletsdispersed in a solvent; and (b) removing at least a portion of solventso as to provide a coated film of at least one layer of MXene plateletsoriented to be essentially coplanar with the substrate surface; saidcoated film being electrically conductive and exhibiting one or morecharacteristics including: (i) a resistivity in a range of from about0.01 to about 1000 micro-ohm-meters; (ii) an ability to transmit atleast about 50% of incident light of at least one wavelength in a rangeof from about 300 nm to about 2500 nm; (iii) a ratio of DC conductivity,measured in Siemens/meter, to light absorbance (including visible lightabsorbance), measured as a decadic absorbance per meter, of at least 0.1Siemens measured at at least one wavelength in the range of 300 to 2500nm; (iv) a value of the real dielectric permittivity of less thannegative one for wavelengths greater than a threshold wavelength in thevisible to near-infrared range; and (v) a combination of any two or moreof (i), (ii), (iii), and (iv).

These coatings may be applied to the rigid or flexible substratesurfaces by spin coating, brushing, dipcoating, or doctor bladingtypically, but not necessarily, using dispersions in water or an organicsolvent, preferably a polar solvent such ethanol, isopropanol,dimethylformaide, pyridine, or dimethylsulfoxide. The coatings may alsocover complete areas or may be patterned using lithographic techniques.

Again, these methods are amenable to a variety of MXene compositions,such as described herein and elsewhere.

Coatings prepared by these inventive methods are also considered withinthe scope of the present invention as are electrical devices andmaterials comprising these coatings. Exemplary devices and materialsinclude, but are not limited to, RFID tag, windows with switchableopacity, photodetectors, liquid crystal displays, light emitting diodes(including organic light emitting diodes), touchscreens, photovoltaicdevices, plasmonic devices such as plasmonic sensor arrays,optical-to-electrical signal transducers, surface-plasmon polaritontransmitters, or an infrared-reflecting window, and metamaterials, suchas cloaking materials, transformational optic components, or superlensescomprising the inventive coatings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present application is further understood when read in conjunctionwith the appended drawings. For the purpose of illustrating the subjectmatter, there are shown in the drawings exemplary embodiments of thesubject matter; however, the presently disclosed subject matter is notlimited to the specific methods, devices, and systems disclosed. Inaddition, the drawings are not necessarily drawn to scale. In thedrawings:

FIGS. 1A-D illustrate a schematic of M_(n+1)X_(n)(T_(s)) clay synthesisand electrode preparation. In FIG. 1A, MAX phase is etched in a solutionof acid and fluoride salt (1), then washed with water to remove reactionproducts and raise the pH toward neutral (2). The resulting sedimentbehaves like a clay; it can be rolled to produce flexible, freestandingfilms (3), molded and dried to yield conducting objects of desired shape(4), or diluted and painted onto a substrate to yield a conductivecoating (5). FIG. 1B shows that when dried samples (left, showing crosssection and top view) are hydrated (right) they swell; upon drying, theyshrink. FIG. 1C shows an actual image of a rolled film. FIG. 1D shows a‘clay’ form shaped to the letter M (˜1 cm) and dried, yielding aconductive solid (inset: experimental conductivity of ‘clay’ rolled to 5μm thickness). The etched material is referred to as Ti₃C₂(T_(s)), wherethe T_(s) stands for surface terminations, such as OH, O and F, whichmay be independently present on one or both surfaces.

FIGS. 2A-F provide structural characterizations of a M_(n+1)X_(n)(T_(s))MXene. FIG. 2A shows XRD patterns of samples produced by etching inLiF/HCl solution. Lower trace: multilayer Ti₃C₂(T_(s)) showing a sharp,intense (0002) and higher order (000l) peaks, corresponding to a clattice parameter of 28 Å and high order in the c direction. Uppertrace: same sample after rolling into a ca. 40 μm thick film; cdirection peaks are preserved, but the prominent (110) peak is no longerobserved, showing significant reduction of order in non-basaldirections. In both cases, traces of Ti₃AlC₂ are still present(diamonds). FIG. 2B shows TEM image of several flakes, showing lateralsizes up to a few hundred nm. Few defective areas are present. Insetshows overall SAED pattern. FIGS. 2C and D shows TEM images of single-and double-layer flakes, respectively; Insets show sketches of theselayers. FIG. 2E shows SEM image of a fracture surface of a ca. 4 μmthick film produced by rolling, showing shearing of layers; flexibilityof the film is demonstrated in inset; FIG. 2F shows a fracture surfaceof a thicker rolled film (ca. 30 μm), showing poorer overall alignmentof flakes in the interior of the film.

FIG. 3A-F show electrochemical performance of rolled, free-standingelectrodes. FIG. 3A shows cyclic voltammetry profiles at different scanrates for a 5 μm thick electrode in 1 M H₂SO₄. FIG. 3B shows acomparison of rate performances reported in this work and previously forHF-produced M_(n+1)X_(n)(T_(s)) MXene. FIG. 3C provides the results of acapacitance retention test of a 5 μm thick rolled electrode in 1 MH₂SO₄. Inset shows galvanostatic cycling data collected at 10 A/g. FIG.3D shows CV profiles collected at 2 mV/s and 20 mV/s with highlightedportions of the contributions of the processes not limited by diffusion;vertical lines limit the CV area used in calculations. FIG. 3E showsrate performance and FIG. 3F shows EIS data, of 5 μm (stars), 30 μm(circles) and 75 μm (triangles) thick rolled electrodes.

FIGS. 4A-F show processing of M_(n+1)X_(n)(T_(s)) clay: FIG. 4A showsthe dried and crushed powder; FIG. 4B and FIG. 4C show the hydrated clayto be plastic and readily formed and molded; FIG. 4D shows films beingproduced in the roller mill; and FIG. 4E and FIG. 4F show rolledfreestanding films being lifted off Celgard membranes.

FIGS. 5A-B show SEM images of FIG. 5A multilayer M_(n+1)X_(n)(T_(s))particle. FIG. 5B cross-section of rolled Ti₃C₂ film, suggesting thatthe shear stresses applied during rolling is most probably responsiblefor the loss of the 60° angle peak in the XRD pattern.

FIGS. 6A-F show TEM characterization of dispersed Ti₃C₂(T_(s)) flakes.FIG. 6A shows a representative TEM image showing the morphology and sizeof a large single-layer Ti₃C₂(T_(s)) flake. Note folding on all sides ofthis large flake. FIG. 6B shows the lateral size distribution of thedispersed Ti₃C₂(T_(s)) flakes. Representative TEM images showing 6Csingle-, 6D, double and 6E, triple-layer flakes. FIG. 6F shows astatistical analysis of layer number distribution of dispersedTi₃C₂(T_(s)) flakes. Note that the fractions of double- and few-layerflakes are overestimated due to inevitable restacking and edge foldingof single-layer flakes during preparation of samples for TEM analysis.The latter is clearly seen in FIG. 6A. An example of the former is shownin FIG. 7.

FIG. 7 shows a TEM image showing the restacking of single- ordouble-layer M_(n+1)X_(n)(T_(s)) flakes into few-layerM_(n+1)X_(n)(T_(s)) MXene.

FIGS. 8A-D shows gravimetrically normalized capacitance. CV profiles atdifferent scan rates for 5 μm (FIG. 8A), 30 μm (FIG. 8B), and 75 μmthick (FIG. 8B) electrodes in 1 M H₂SO₄. FIG. 8D shows gravimetric rateperformances of rolled electrodes: 5 μm (squares), 30 μm (circles) and75 μm (triangles) thick.

FIGS. 9A and 9B show some of the optical and electrical characteristicsof exemplary spincoated MXene films. FIG. 9A shows the UV-vis-NIRtransmittance spectra of a set of spincoated MXene films of varyingthickness. FIG. 9B shows the sheet resistance vs. transmittance of MXenefilms spincast on soda-lime glass. Transmittance values represent theaverage transmittance of light between 550 nm and 1100 nm. The trendline is an exponential fit for points with transmittance between 2% and85%. Error bars represent the standard deviation of the sheet resistanceover the film and are present on all data points, but are too small tobe seen in some cases. Upper, MXene films with transmittance rangingfrom 74.8% (left) to 91.7% (right). Lower, spincast MXene film with atransmittance of 30%.

FIGS. 10A, 10B, and 10C show film thickness and surface characterizationthrough microscopy. FIG. 10A is an SEM micrograph of the profile of acleaved MXene film spincoated on a silicon wafer. FIG. 10B is a surfaceSEM micrograph of a spincoated MXene film. FIG. 10C shows therelationships of transmittance vs AFM-measured thickness of MXene films.A linear fit gave the absorption coefficient, α=1.42×10⁵ cm⁻¹, R²=0.996.Inset: Example AFM measurement to determine film thickness. A scratchwas made in the MXene film on silicon, shown here for a film with 7%transmittance. The left side shows the surface of the MXene film, andthe right shows the scratch. The film was 78 nm thick with a surfaceroughness of 8.7 nm.

FIG. 11 shows the conductivity of exemplary spincoated MXene films.Inverse sheet resistance vs thickness of MXene films. Thickness valueswere calculated from the measured transmittance, using the calibrationby AFM. The slope of the linear fit provides the conductivity, σ=6.8×10³S/cm (R²=0.93). Inset is an expanded view of the thinnest films.

FIGS. 12A and 12B show data for spectroscopic ellipsometry of exemplaryspincoated MXene films. FIG. 12A shows the complex reflectivity values,Ψ (top three curves) and A (bottom three curves), as a function ofwavelength, collected at 50°, 60°, and 70° , of spincoated MXene films.An optical model consisting of a Drude oscillator, and two harmonicoscillators, provides a mean-square-error of 9.91 over the entire rangeof wavelengths and angles. The Drude oscillator indicates a resistivityof 1.36×10⁴ Ω·cm and scattering time of 1.73 fs. FIG. 12A shows the realand imaginary dielectric constants as a function of wavelength. Themodels used to develop FIGS. 12A and 12B were the same.

FIGS. 13A and 13B show examples of spincoated MXene film stability. FIG.13A shows the relative changes in sheet resistance vs. time forspincoated MXene films stored in open air (solid lines) and analogousfilms stored under nitrogen (dashed lines). Red lower, green middle, andblue upper lines within each category represent films with transmittance(thickness) of 27% (92 nm), 53% (45 nm), and 82% (14 nm) respectively.FIG. 13B shows sheet resistance of three spincoated MXene films in asequence of wetting and drying. Films were measured as spincoated(leftmost in each grouping), then after being stored under dry nitrogenfor overnight (second from left in each grouping), stored in wetnitrogen for two days (second from right in each grouping), and lastlystored under dry nitrogen for one day (rightmost in each grouping)

FIGS. 14A, 14B, and 14C show some of the structural and electricalproperties of exemplary spincoated MXene films by different techniquesand on different substrates. FIG. 14A shows a spincoated MXene film on aflexed polyetherimide substrate. FIG. 14B shows an XRD pattern of anexemplary spincoated MXene film on glass, showing 002 peak at 7.1°corresponding to c lattice parameter of 25 Å. FIG. 14C shows a severalmeasures of conductivity of spincoated MXene films measured. Theconductivity of MXene films on the polymer substrate was measured bytwo-point resistivity, translated into bulk conductivity using thethickness extrapolated from the measured transmittance and thecross-sectional area of the electrodes.

FIG. 15 shows a patterned MXene film, masked by adhesive tape stripsafter developing the exposed regions in nitric acid. The exposed MXeneis oxidized to transparency.

FIG. 16 shows XRD patterns of Ti₃C₂-2 as washed initially (bottomtrace), after washing with HCl (middle dotted trace,), and aftersubsequent washing with LiCl to yield Li—Ti₃C₂T_(x) (solid top trace,top). Vertical dashed lines denote peaks from crystalline Si used as astandard, and solid black circles denote LiF impurities present beforewashing with HCl. See Example 8.

FIG. 17A shows overlaid in situ XRD patterns of hydrated Li-Ti₃C₂T_(x)upon drying in 40% relative humidity. FIG. 17B shows a contour plotshowing the time evolution of the data shown in FIG. 17A. FIG. 17C showsthe basal spacings from the (0002) peaks over time, showing an abruptchange in d-spacing and the coexistence of monolayer and bilayer phases.FIG. 17D shows changes in intensity with time of (0002) peak ofmonolayer (red triangles) and bilayer (blue squares) phases. See Example8.

FIGS. 18A-18C show XRD patterns of ion-exchanged Ti₃C₂T_(x) undervarious conditions. FIG. 18A shows the rapid rehydration with liquidwater; FIG. 18B shows the traces for samples equilibrated for 48 h at˜50% relative humidity; FIG. 18C shows the XRD traces for samples driedover anhydrous P₂O₅ for 48 h. FIGS. 18D-F show the change in d-spacingper interlayer space extracted from XRD data in (a-c) as a function ofe/r, where e is the charge on the cation and r its radius. d) rapidrehydration: Ti₃C₂ (triangles), with ion-intercalatedA_(0.5)(H₂O)y[TiS₂]^(0.5), where A is a cation (open circles) taken fromreference 12. FIG. 18E shows the XRD traces for Ti₃C₂T_(x) equilibratedfor 48 h at ˜50% relative humidity. FIG. 18F shows the XRD traces forTi₃C₂T_(x) dried over anhydrous P₂O₅ for 48 h. The patterns in a, b, andc are colour-coordinated with the labelled cations. FIG. 18G showscartoons depicting the interlayer space with cation intercalationleading to a single layer of H₂O (top) or a two-layer arrangement(bottom). Green: Ti; black: C; purple: generic surface termination(O/OH/F); red: O; white: H; teal: intercalated cation. See Example 8.

FIGS. 19A-19C show XPS spectra with curve-fitting for: FIG. 19A: Li 1 sregion for (i) Ti₃C₂T_(x)-1 (ii) Li—Ti₃C₂T_(x) (iii) Na—Ti₃C₂T_(x) (iv)Rb—Ti₃C₂T_(x) before sputtering, dashed vertical lines represent, fromleft to right, species LiF/LiCl and LiOH/Li₂O, and the large shoulder onthe left is due to the Ti 3s peak, FIG. 19B: Na is region for (i)Na—Ti₃C₂T_(x) before sputtering, and, (ii) Na—Ti₃C₂T_(x) aftersputtering. Dashed vertical lines, from left to right, represent thespecies NaOH (Na 1s), NaF/NaCl (Na 1s), Ti—C (Auger LMM line), and TiO₂(Auger LMM line), and FIG. 19C: Rb 3d region for (i) Rb—Ti₃C₂T_(x)before sputtering, and (ii) Rb—Ti₃C₂T_(x) after sputtering. Dashedvertical lines, from right to left, represent the species Rb⁺(3d_(5/2)), RbCl (3d_(5/2)), Rb⁺ (3d_(3/2)), and RbCl (3d_(3/2)). SeeExample 8.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing description taken in connection with the accompanying Figuresand Examples, all of which form a part of this disclosure. It is to beunderstood that this invention is not limited to the specific products,methods, conditions or parameters described and/or shown herein, andthat the terminology used herein is for the purpose of describingparticular embodiments by way of example only and is not intended to belimiting of any claimed invention. Similarly, unless specificallyotherwise stated, any description as to a possible mechanism or mode ofaction or reason for improvement is meant to be illustrative only, andthe invention herein is not to be constrained by the correctness orincorrectness of any such suggested mechanism or mode of action orreason for improvement. Throughout this text, it is recognized that thedescriptions refer both to the methods of preparing the desiredproducts, as well as the use of the products so prepared, and viceversa.

Terms

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art, and so forth.

When a value is expressed as an approximation by use of the descriptor“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function. The person skilledin the art will be able to interpret this as a matter of routine. Insome cases, the number of significant figures used for a particularvalue may be one non-limiting method of determining the extent of theword “about.” In other cases, the gradations used in a series of valuesmay be used to determine the intended range available to the term“about” for each value. Where present, all ranges are inclusive andcombinable. That is, references to values stated in ranges include everyvalue within that range.

When a list is presented, unless stated otherwise, it is to beunderstood that each individual element of that list and everycombination of that list is to be interpreted as a separate embodiment.For example, a list of embodiments presented as “A, B, or C” is to beinterpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A orC,” “B or C,” or “A, B, or C.”

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.That is, unless obviously incompatible or specifically excluded, eachindividual embodiment is deemed to be combinable with any otherembodiment(s) and such a combination is considered to be anotherembodiment. Conversely, various features of the invention that are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any sub-combination. It is further noted thatthe claims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for use of suchexclusive terminology as “solely,” “only” and the like in connectionwith the recitation of claim elements, or use of a “negative”limitation. Finally, while an embodiment may be described as part of aseries of steps or part of a more general structure, each said step orpart may also be considered an independent embodiment in itself

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present invention, representativeillustrative methods and materials are described herein.

In this specification and in the claims that follow, reference will bemade to a number of terms, which shall be defined to have the followingmeanings.

Supplementing the descriptions herein, M_(n+1)X_(n)(T_(s)) (includingM′₂M″_(m)X_(m+1)(T_(s)) compositions) may be viewed as comprising freestanding and stacked assemblies of two dimensional crystalline solids.Collectively, such compositions are referred to herein as“M_(n+1)X_(n)(T_(s)),” “MXene,” “MXene compositions,” or “MXenematerials.” Additionally, these terms “M_(n+1)X_(n)(T_(s)) ,” “MXene,”“MXene compositions,” or “MXene materials” can also independently referto those compositions derived by the chemical exfoliation of MAX phasematerials, whether these compositions are present as free-standing2-dimensional or stacked assemblies (as described further below). MXenecompositions comprise at least one layer having first and secondsurfaces, each layer comprising: a substantially two-dimensional arrayof crystal cells; each crystal cell having an empirical formula ofM_(n+1)X_(n), where M, X, and n are defined herein. These compositionsmay be comprised of individual or a plurality of such layers. In someembodiments, the MXenes comprising stacked assemblies may be capable of,or have atoms, ions, or molecules, that are intercalated between atleast some of the layers. In other embodiments, these atoms or ions arelithium. In still other embodiments, these structures are part of anenergy-storing device, such as a battery or supercapacitor.

The term “crystalline compositions comprising at least one layer havingfirst and second surfaces, each layer comprising a substantiallytwo-dimensional array of crystal cells” refers to the unique characterof these materials. For purposes of visualization, the two-dimensionalarray of crystal cells may be viewed as an array of cells extending inan x-y plane, with the z-axis defining the thickness of the composition,without any restrictions as to the absolute orientation of that plane oraxes. It is preferred that the at least one layer having first andsecond surfaces contain but a single two-dimensional array of crystalcells (that is, the z-dimension is defined by the dimension ofapproximately one crystal cell), such that the planar surfaces of saidcell array defines the surface of the layer; it should be appreciatedthat real compositions may contain portions having more than singlecrystal cell thicknesses.

That is, as used herein, “a substantially two-dimensional array ofcrystal cells” refers to an array which preferably includes a lateral(in x-y dimension) array of crystals having a thickness of a single unitcell, such that the top and bottom surfaces of the array are availablefor chemical modification.

High Capacitance “Clays”

Various embodiments of the present disclosure include those comprising:(a) adjusting the water content of a M_(n+1)X_(n)(T_(s)) composition toform a compressible paste wherein the ratio of water toM_(n+1)X_(n)(T_(s)) MXene by mass is in a range of from about 0.3 toabout 0.65; and (b) compressing an amount of the MXene composition witha pressure of at least 5 psig to form a solid body that is electricallyconductive exhibiting (i) a resistivity in a range of from about 1 toabout 10,000 micro-ohm-meters; (ii) volumetric capacity of at least 500F/cm³ when tested as a scan rate of 20 mV/s or at least 550, 600, 650,700, 750, or 800 F/cm³ when tested as a slower scan rate; or (iii) both(i) and (ii). M_(n+1)X_(n)(T_(s)) compositions are more fully describedherein, but may be defined here as comprising at least one layer(preferably a plurality of layers) each having a first and secondsurface, each layer comprising a substantially two-dimensional array ofcrystal cells; each crystal cell having an empirical formula ofM_(n+1)X_(n), such that each X is positioned within an octahedral arrayof M, wherein M is at least one Group 3, 4, 5, 6, or 7 metal(corresponding to Group IIIB, IVB, VB, VIB, or VIIB metal), wherein eachX is C, N, or a combination thereof and n=1, 2, or 3; wherein at leastone of said surfaces of the layers has surface terminations, T_(s),independently comprising alkoxide, alkyl, carboxylate, halide,hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide,sulfonate, thiol, or a combination thereof.

Various other embodiments provide that the ratio of water toM_(n+1)X_(n)(T_(s)) MXene is in a range bounded at the lower end by avalue of about 0.1, 0.2, 0.25, 0.35, 0.4, 0.45, and 0.5 and is boundedby a value at the higher end by a value of about 0,65, 0.5, 0.45, or0.4. A range of 0.3 to about 0.65 provides a good range for easyhandling of the materials, but compositions at the lower end of thewater range are necessarily formed during the processing of theconductive materials. While the compositions having these water to MXeneratios are described in terms of the method described herein, it shouldbe appreciated that the compositions themselves are also consideredseparate embodiments of this disclosure.

Compositions in the ranges described are characterized by their abilityto reversibly swell in volume with addition or removal of water (bothobservable by the naked eye as volume change and observable via x-raydiffraction on the unit-cell level). The extent of the water present inthe structure can be controlled by controlling the addition of water tothe solids or even by controlling the humidity of the environment.M_(n+1)X_(n)(T_(s)) clays within these ranges may be characterized asmalleable on handling; i.e., having a consistency of modeling clay.

Where the ratio of water to MXene is above 0.65, the mixtures are betterdescribed as a colloidal dispersion ‘ink.’ Such inks can be deposited(e.g., by gravitational settling or vacuum assisted filtration) anddried (e.g., by evaporation) to yield MXene films. Such films can bemade thin enough to be optically transparent while retaining highconductivity. The film thickness and optical properties (transparency,interference colors) can be controlled by varying the concentration ofMXene in the applied suspension.

While the application of pressure is described in terms of pressuresexceeding 5 psig, this value is flexible, and the compressing can bedone at a pressure in a range of from about 5 psig to about 500 psig. Insome embodiments, the compressing is accompanied by the removal ofwater, as water is squeezed from the composition, and so it is sometimeshelpful to provide adsorbant materials between the MXene compositionsand the compression, or other means of removing water.

The compression can be accomplished by any conventional methods known inthe art (using conventional flattening or shaping equipment), but ismost conveniently done by compression molding or rolling, for example byrolling the M_(n+1)X_(n)(T_(s)) composition between at least two rollerbars. See, e.g., FIG. 1A. The compression may be applied in the presenceor absence of heat (for example, up to about 50° C.), e.g., using heatedor unheated platens or platen-like devices.

Again, it should be apparent that the thickness of the final solid bodydepends on a number of parameters, including initial pre-compressionloading, water content, and pressure; it is convenient to describe thesesolid bodies as being compressed to a thickness in a range of from about0.1 microns to about 1000 microns. Independent embodiments provide thatthe solid body thicknesses can range from about 0.1 to about 0.5microns, from about 0.5 to about 1 microns, from about 1 to about 2microns, from about 2 to about 3 microns, from about 3 to about 4microns, from about 4 to about 5 microns, from about 5 to about 10microns, from about 10 to about 20 microns, from about 20 to about 30microns, from about 30 to about 40 microns, from about 40 to about 50microns, from about 50 to about 75 microns, from about 75 to about 100microns, from about 100 to about 500 microns, from about 500 to about1000 microns, or any combination thereof. While these solid bodies arebinder-free as prepared, additives and binders may be added as otherwisedesired.

The physical form of the MXenes used in these methods may be at leastpartly defined by the way in which they are prepared. The preferredmethod of preparing M_(n+1)X_(n)(T_(s)) compositions is the reactivedelamination of MAX-phase materials, the method resulting in flakes ofM_(n+1)X_(n)(T_(s)) materials. In particular, the reaction of precursorMAX phase materials with the milder sources of HF described herewithintends to yield larger M_(n+1)X_(n)(T_(s)) flakes on the reactivedelamination than are available when HF alone is used for this purpose.Accordingly, in some embodiments, the M_(n+1)X_(n)(T_(s)) compositioncomprises a plurality of M_(n+1)X_(n)(T_(s)) flakes having at least onemean lateral dimension in a range of from about 0.5 micron to about 10microns.

The compositions prepared by these methods provide solid bodies whichare electrically conductive, wherein the solid body has surfaceelectrical resistance in a range of from about 1 micro-ohm-meters toabout 10,000 micro-ohm-meters. In other embodiments, the surfaceresistivity is in a range of from about 1 micro-ohm-meters to about 10micro-ohm-meters, from about 10 micro-ohm-meters to about 100micro-ohm-meters, from about 100 micro-ohm-meters to about 1000micro-ohm-meters, from about 1000 micro-ohm-meters to about 10,000micro-ohm-meters, or a combination thereof

Ångstrom-Thin Conducting Films

Other various embodiments of the present disclosure include thosemethods for preparing thin conducting films of MXene materials, thesemethods comprising: (a) applying a MXene dispersion onto a substratesurface, said MXene dispersion comprising or (consisting essentially of)at least one type of MXene platelets dispersed in a solvent; and (b)removing at least a portion of solvent so as to provide a coated film ofat least one layer of MXene platelets oriented to be essentiallycoplanar with the substrate surface,

-   said coated film being electrically conductive and exhibiting:

(i) a resistivity in a range of from about 0.01 to about 1000micro-ohm-meters;

(ii) an ability to transmit at least about 50%, at least about 55%, atleast about 60%, at least about 65%, at least about 70%, at least about75%, at least about 80%, at least about 85%, at least about 90%, or atleast about 95% of incident light of at least one wavelength in a rangeof from about 300 nm to about 2000 nm;

(iii) a ratio of DC conductivity, measured in Siemens/meter, to lightabsorbance, (including visible light absorbance), measured as a decadicabsorbance per meter, of at least 0.1 Siemens measured at at least onewavelength in the range of 300 to 2500 nm;

(iv) a value of the real dielectric permittivity of less than negativeone for wavelengths greater than a threshold wavelength, for example,500 nm; or

(v) a combination of any two or more of (i), (ii), (iii), and (iv).

Any individual material may exhibit one, two, or more of these features.Again, the M_(n+1)X_(n)(T_(s)) compositions are more fully describedelsewhere, but may be defined here as comprising at least one layer(preferably a plurality of layers) each having a first and secondsurface, each layer comprising a substantially two-dimensional array ofcrystal cells; each crystal cell having an empirical formula ofM_(n+1)X_(n), such that each X is positioned within an octahedral arrayof M, wherein M is at least one Group 3, 4, 5, 6, or 7 metal(corresponding to Group IIIB, IVB, VB, VIB, or VIIB metal), wherein eachX is C, N, or a combination thereof and n=1, 2, or 3; wherein at leastone of said surfaces of the layers has surface terminations, T_(s),independently comprising alkoxide, alkyl, carboxylate, halide,

In certain sub-embodiments, the coated film, whether prepared byspincoating or otherwise, may independently exhibit a surfaceresistivity in a range bounded at the lower end by a value of about0.01, 0.1, 1, 5, 10, 50, 100, 250, or 500 micro-ohm-meters and at theupper end by a value of about 1000, 500, 100, 50, 25, 10, or 5micro-ohm-meters, for example in a range of from about 1 to about 10micro-ohm-meters. Alternatively, the surface resistivity of the coatingsmay be described in terms of being in a range of from about 0.01micro-ohm-meters to about 0.1 micro-ohm-meters, from about 0.1micro-ohm-meters to about 1 micro-ohm-meters, from about 1micro-ohm-meters to about 10 micro-ohm-meters, from about 10micro-ohm-meters to about 100 micro-ohm-meters, from about 100micro-ohm-meters to about 1000 micro-ohm-meters, from about 1000micro-ohm-meters to about 10,000 micro-ohm-meters, or any combination oftwo or more of these ranges.

In certain sub-embodiments, the coated film, whether prepared byspincoating or otherwise, may independently exhibit the described lighttransmittance of at least one wavelength in a range of from about 300 nmto about 400 nm, from about 400 nm to about 500 nm, from about 500 nm toabout 600 nm, from about 600 nm to about 700 nm, from about 700 nm toabout 800 nm, from about 800 nm to about 900 nm, from about 900 nm toabout 1000 nm, from about 1000 nm to about 1200 nm, from about 1200 nmto about 1400 nm, from about 1400 nm to about 1600 nm, from about 1600nm to about 1800 nm, from about 1800 nm to about 2000 nm, from about2000 nm to about 2500 nm, or any combination of two or more of theseranges, for example from about 300 nm to about 800 nm.

While not always the case, it has been observed that sheet resistivitiestend to increase with increasing transparency, as shown in FIG. 9B. Insome embodiments, this relationship may be described in terms of a ratioof DC conductivity, measured in Siemens/meter, to light absorbance(including visible light absorbance), measured as a decadic absorbanceper meter, of at least 0.1 and up to about 300 Siemens measured at atleast one wavelength in any one of the ranges of light listed above.

In some embodiments, the coated MXene films, whether prepared byspincoating or otherwise, can exhibit surface conductivities in a rangeof from about 100 to 500 S/cm, from 500 to 1000 S/cm, from 1000 to 2000S/cm, from 2000 to 3000 S/cm, from 3000 to 4000 S/cm, from 4000 to 5000S/cm, from 5000 to 6000 S/cm, from 6000 to 7000 S/cm, from 7000 to 8000S/cm, or any combination of two or more of these ranges. Suchconductivities may be seen on flat or flexed substrates.

The coatings exhibit complex dielectric permittivities having real andimaginary parts (see FIG. 12B). As normally found for such complexpermittivities, those of the present coatings are a complicated functionof frequency, co, since it is a superimposed description of dispersionphenomena occurring at multiple frequencies. Generally, at a givenfrequency, a negative real permittivity leads to reflection of incidentlight. In certain embodiments of the present invention, the real part ofthe complex permittivities exhibited by the coatings have values of lessthan negative one for wavelengths greater than a threshold wavelength,for example, 1130 nm. This is a requirement for plasmonic applicationsas it indicates a coupling of the free electrons (the electron plasma)of the material with an incident light field.

Typically, the dispersions are prepared in aqueous or organic solvents.In addition to the presence of the MXene materials, aqueous dispersionsmay also contain processing aids, such as surfactants, or ionicmaterials, for example lithium salt or other intercalating orintercalatable materials. If organic solvents are used, polar solventsare especially useful, including alcohols, amides, amines, orsulfoxides, for example comprising ethanol, isopropanol,dimethylacetamide, dimethylformamide, pyridine, and/ordimethylsulfoxide.

It is convenient to apply the MXene dispersions by any number ofindustry recognized methods for depositing thin coatings on substrates,depending on the viscosity of the dispersion. This viscosity may dependon the concentration of MXene particles or sheets in the dispersion, aswell as the presence and concentrations of other constituents. Forexample, at MXene concentrations of between 0.001 and 100 mg/mL, it isconvenient to apply the MXenes to the substrate surface by spin coating.In some embodiments, these dispersions are applied dropwise onto the anoptionally rotating substrate surface, during or after which thesubstrate surface is made to rotate at a rate in a range of from about300 rpm (rotations per minute) to about 5000 rpm. Rotational speeddepends on a number of parameters, including viscosity of dispersion,volatility of the solvent, and substrate temperature as are understoodby those skilled in the art.

Other embodiments provide that the MXene dispersions are areally appliedto the substrate surface (i.e., over an extended area of the substrate),for example by brushing, dipcoating, spray coating, or doctor blading.These films may be allowed to settle (self-level) as stationary films,but in other embodiments, these brushed, dipcoated, or doctor bladedfilms may be also subjected to rotating the substrate surface at a ratein a range of from about 300 rpm to about 5000 rpm. Depending on thecharacter of the dispersions, this may be used to level or thin thecoatings, or both.

Once applied, at least a portion of the solvent is removed or lost byevaporation. The conditions for this step obviously depend on the natureof the solvent, the spinning rate and temperature of the dispersion andsubstrate, but typically convenient temperatures include those in arange of from about 10° C. to about 300° C., though processing thesecoatings is not limited to these temperatures.

Additional embodiments provide that multiple coatings may be applied,that that the resulting coated film comprises an overlapping array oftwo or more overlapping layers of MXene platelets oriented to beessentially coplanar with the substrate surface.

Similarly, the methods are versatile with respect to substrates. Rigidor flexible substrates may be used. Substrate surfaces may be organic,inorganic, or metallic, and comprise metals (Ag, Au, Cu, Pd, Pt) ormetalloids; conductive or non-conductive metal oxides (e.g., SiO₂, ITO),nitrides, or carbides; semi-conductors (e.g., Si, GaAs, InP); glasses,including silica or boron-based glasses; liquid crystalline materials;or organic polymers. Exemplary substrates include metallized substrates;oxidizes silicon wafers; transparent conducting oxides such as indiumtin oxide, fluorine doped tin oxide, aluminum-doped zinc-oxide (AZO),indium-doped cadmium-oxide, or aluminum, gallium or indium-doped zincoxide (AZO, GZO or IZO); photoresists or other organic polymers. TheseMXene coatings may be applied to flexible substrates as well, includingorganic polymer materials. Exemplary organic polymers include thosecomprising polyetherimide, polyetherketone, polyetheretherketone,polyamide; exemplary liquid crystal materials include, for example,poly(3,4-ethylenedioxythiophene) [PEDOT] and its derivatives; organicmaterials can also be photosensitive photoresists

Flat surface or surface-patterned substrates can be used. For example,the substrate surface may comprise recesses, such as channels orvias/holes, or protrusions, such as pillars, posts, or walls, forexample forming lines in any channels or between walls. The MXenecoatings may also be applied to surfaces having patterned metallicconductors or nanoparticles, for example nanotubes or nanowires,including carbon nanotubes or nanowires. Additionally, by combiningthese techniques, it is possible to develop patterned MXene layers byapplying a MXene coating to a photoresist layer, either a positive ornegative photoresist, photopolymerize the photoresist layer, and developthe photopolymerized photoresist layer. During the developing stage, theportion of the MXene coating adhered to the removable portion of thedeveloped photoresist is removed. Alternatively, or additionally, theMXene coating may be applied first, followed by application, processing,and development of a photoresist layer. By selectively converting theexposed portion of the MXene layer to an oxide using nitric acid, aMXene pattern may be developed. The exposed MXene is transformed intotitanium dioxide by nitric acid, while the masked portion retains itsconductivity (See FIG. 15). Still further, it is possible to applypatterned conductors or functional elements to these patterned MXenecoatings, using conventional methods, such as sputtering, e-beamlithography, etc. In short, these MXene materials may be used inconjunction with any appropriate series of processing steps associatedwith thick or thin film processing to produce any of the structures ordevices described herein (including, e.g., plasmonic nanostructures).

MXene Materials

As described elsewhere within this disclosure, the M_(n+1)X_(n)(T_(s))materials used in these methods and compositions have at least onelayer, and sometimes a plurality of layers, each layer having a firstand second surface, each layer comprising a substantiallytwo-dimensional array of crystal cells; each crystal cell having anempirical formula of M_(n+1)X_(n), such that each X is positioned withinan octahedral array of M, wherein M is at least one Group 3, 4, 5, 6, or7 metal (corresponding to Group IIIB, IVB, VB, VIB or VIIB metal),wherein each X is C, N, or a combination thereof and n=1, 2, or 3;wherein at least one of said surfaces of the layers has surfaceterminations, T_(s), comprising alkoxide, alkyl, carboxylate, halide,hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide,sulfonate, thiol, or a combination thereof

Metals of Group 3, 4, 5, 6, or 7 (corresponding to Group IIIB, IVB, VB,VIB, or VIIB), either alone or in combination, said members includingSc, Y, Lu, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W. For the purposes ofthis disclosure, the terms “M” or “M atoms,” “M elements,” or “M metals”may also include Mn.

In some embodiments, the M is at least one Group 4, 5, or 6 metal or Mn.In preferred embodiments, M is one or more of Hf, Cr, Mn, Mo, Nb, Sc,Ta, Ti, V, W, or Zr, or a combination thereof. In other preferredembodiments, the transition metal is one or more of Ti, Zr, V, Cr, Mo,Nb, Ta, or a combination thereof. In even more preferred embodiments,the transition metal is Ti, Ta, Mo, Nb, V, Cr, or a combination thereof

In certain specific embodiments, M_(n+1)X_(n) comprises Sc₂C, Sc₂N,Ti₂C, Ti₂N, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N, Hf₂C, Hf₂N,Ta₂C, Mo₂C, Ti₃C₂, Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂, Mo₃C₂, (Cr_(2/3)Ti_(1/2))₃C₂, Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃, Ta₄N₃, Nb₄C₃, or acombination thereof.

Preferred precursor MAX phase materials include those wherein M is atleast one of Hf, Cr, Mn, Mo, Nb, Ta, Ti, V, W, or Zr. Other preferredembodiments include those where the A in the MAX phase material is atleast one of Al, As, Ga, Ge, In, P, Pb, S, or Sn.

In more specific embodiments, the M_(n+1)X_(n) (T_(s)) crystal cellshave an empirical formula Ti₃C₂ or Ti₂C and at least one of saidsurfaces of each layer is coated with surface terminations, T_(s),comprising alkoxide, fluoride, hydroxide, oxide, sub-oxide, sulfonate,or a combination thereof.

The range of compositions available can be seen as extending evenfurther when one considers that each M-atom position within the overallM_(n+1)X_(n) matrix can be represented by more than one element. Thatis, one or more type of M-atom can occupy each M-position within therespective matrices. In certain exemplary non-limiting examples, thesecan be (M^(A) _(x)M^(B) _(y))₂C or (M^(A) _(x)M^(B) _(y))₂N, (M^(A)_(x)M^(B) _(y))₃C₂ or (M^(A) _(x)M^(B) _(y))₃N₂, or (M^(A) _(x)M^(B)_(y))₄C₃ or (M^(A) _(x)M^(B) _(y))₄N₃, where M^(A) and M^(B) areindependently members of the same group, and x+y=1. For example, in butone non-limiting example, such a composition can be(V_(1/2)Cr_(1/2))₃C₂. In the same way, one or more type of X-atom canoccupy each X-position within the matrices, for example solid solutionsof the formulae M_(n+1)(C_(x)N_(y))_(n), or (M^(A) _(x)M^(B)_(y))_(n+1)(C_(x)N_(y))_(n).

In more specific embodiments, the MXenes may comprise compositionshaving at least two Group 4, 5, 6, or 7 metals, and theM_(n+1)X_(n)(T_(s)) composition is represented by a formulaM′₂M″_(m)X_(m+1)(T_(s)), where m=1 or 2 (where m=n−1, in the context ofthe general MXene formula. Typically, these are carbides (i.e., X iscarbon). Such compositions are described in U.S. Patent Application No.62/149,890, this reference being incorporated herein by reference forall purposes. In these double transition metal carbides, M′ may be Ti,V, Cr, or Mo. In these ordered double transition metal carbides, M″ maybe Ti, V, Nb, or Ta, provided that M′ is different than M″. Thesecarbides may be ordered or disordered. Individual embodiments of theordered double transition metal carbides include those compositionswhere M′₂M″_(m)X_(m+1), is independently Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂,Mo₂NbC₂, Mo₂Ti₂C₃, Cr₂TiC₂, Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂,V₂TaC₂, V₂TiC₂, or a combination thereof. In some other embodiments,M′₂M″_(m)X_(m+1), is independently Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂,Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, V₂TiC₂, or acombination thereof. In other embodiments, M′₂M″_(m)X_(m+1), isindependently Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃, Cr₂Ti₂C₃, Cr₂V₂C₃,Cr₂Nb₂C₃, Cr₂Ta₂C₃, Nb₂Ta₂C₃, Ti₂Nb₂C₃, Ti₂Ta₂C₃, V₂Ta₂C₃, V₂Nb₂C₃,V₂Ti₂C₃, or a combination thereof. In still other embodiments,M′₂M″_(m)X_(m+1), is independently Nb₂VC₂, Ta₂TiC₂, Ta₂VC₂, Nb₂TiC₂ or acombination thereof.

These MXene materials, described above as either M_(n+1)X_(n)(T_(s)) orM′₂M″_(m)X_(m+1), may be prepared by selectively removing an A groupelement from a precursor MAX-phase material. Depending on the specificMAX being considered, these A group elements may be independentlydefined as including Al, As, Cd, Ga, Ge, P, Pb, In, S, Si, Sn, or Tl.Some of these A-group elements may be removed in aqueous media, forexample, by a process comprising a treatment with a fluorine-containingacid. For example, Al, As, Ga, Ge, In, P, Pb, S, or Sn may be removed inthis way, although Al is especially amenable to such extractions.Aqueous hydrofluoric acid is particularly suitable for this purpose,whether used as provided, or generated in situ by other conventionalmethods. Such methods include the use of any one or more of thefollowing:

-   (a) aqueous ammonium hydrogen fluoride (NH₄F·HF);-   (b) an alkali metal bifluoride salt (i.e., QHF₂, where Q is Li, Na,    or K), or a combination thereof; or-   (c) at least one fluoride salt, such as an alkali metal, alkaline    earth metal, or ammonium fluoride salt (e.g., LiF, NaF, KF, CsF,    CaF₂, tetraalkyl ammonium fluoride (e.g., tetrabutyl ammonium    fluoride)) in the presence of at least one mineral acid that is    stronger than HF (i.e., has a higher Ka value) and can react with    fluorides to form HF in situ (such as HCl, HBr, HI, H₃PO₄, HNO₃,    oxalic acid, or H₂SO₄); or-   (d) a combination of two or more of (a)-(c).

In specific embodiments, the fluorine-containing acid is derived fromlithium fluoride and a strong aqueous mineral acid, such as HCl, HNO₃,or H₂SO₄, preferably HCl.

It also appears that the use of aqueous HF in the presence of one ormore alkali halides, such as LiCl, provides advantages over using HFalone, or by reacting LiF with aqueous HCl. The use of LiF with aqueousHCl avoids the handling issues associated with the use of aqueous HF andprovides higher yields of single-layer flakes, in some cases it may bedifficult to remove LiF impurities and the removal of the A-element(e.g., Al) is slower. The use of LiCl with aqueous HF provides morecrystalline MXene phases, with better control of the basal spacing (cparameter) and it is easier to vary the procedures especially for thoseinvolving ion intercalation.

Perhaps at least as importantly, the use of mixtures of alkali metal oralkaline earth metal salts (typically chlorides or bromides) incombination with HF during the preparation of the MXene materials (e.g.,using LiCl, NaCl, KCl, KBr, RbCl, MgCl₂, CaCl₂ with aqueous HF) providesnew and unexpected opportunities for the intercalation of these metalcations (hydrated or otherwise) into the MXene matrices. Theseopportunities are more fully described in Example 8. It should beappreciated that, while the studies described in Example 8 focused onTi₃C₂ materials, and all of the descriptions are considered specificembodiments of this invention, the invention is not limited by theseexamples, and additional embodiments provide for the intercalation ofthese metal ions in the full array of MXene materials described herein.

The conditions of the disclosed methods provide solid bodies exhibitingvolumetric capacitances in a range of about 500 F/cm³ to about 1500F/cm³, or in a range of from about 100 F/g to about 500 F/g, when testedas a scan rate of 2 mV/s.

These methods may also be used to prepare MXene composition comprisingplatelets having at least one mean lateral dimension in a range of fromabout 0.1 micron to about 50 micrometers, which are especiallyattractive for the coatings described herein.

Compositions and Devices

While described thus far in terms of methods, it should be appreciatedthat the present disclosure embraces those embodiments comprising thesolid bodies or films prepared by any one of the disclosed methods.These embodiments includes those solid bodies so prepared, furthercomprising intercalated lithium or other ions (such as alkali oralkaline earth or transition metal ions). These solid bodies may exhibitany of the electrical properties described above.

Additional embodiments considered within the scope of this disclosureinclude electrodes or other electrochemical devices, including thoselisted below, and especially batteries and supercapacitors, comprisingany of these solid bodies.

Additional embodiments also include the MXene coatings as describedherein, as well as materials and electrical devices incorporating thesecoatings. These materials may contain any of the intercalatedderivatives previously described for these materials, including forexample intercalated lithium ions, lithium atoms, or a combination oflithium ions and lithium atoms. Such compositions are described inco-pending U.S. patent application Ser. No. 14/094,966, which isincorporated by reference herein at least for this teaching.

The coatings may be incorporated or used in a wide variety of electricaldevices and each of these are considered within the scope of thisinvention. Such independent exemplary devices include, but are notlimited to RFID tags, windows with switchable opacity, light emittingdiodes (including organic light emitting diodes), touchscreens,photovoltaics, photodetectors, liquid crystal displays, touchscreens, orphotovoltaic devices. Liquid-crystal displays (LCDs) may be consideredin terms of a flat panel displays, electronic visual displays, and/orvideo displays that use the light modulating properties of liquidcrystals. Light emitting diodes (including organic light emittingdiodes) may be used to create digital displays in devices such astelevision screens, computer monitors, portable systems such as mobilephones, handheld game consoles, and PDAs.

Other independent examples include plasmonic devices such as plasmonicsensor arrays, optical-to-electrical signal transducers, asurface-plasmon polariton transmitters, and infrared-reflecting windows.In other embodiments, the inventive coatings are used to preparemetamaterials such as cloaking materials, transformational opticcomponents, and superlenses, and each are considered within the presentscope.

ADDITIONAL EMBODIMENTS

The following listing of embodiments in intended to complement, ratherthan displace or supersede, any of the previous descriptions.

Embodiment 1

A method comprising: (a) adjusting the water content of aM_(n+1)X_(n)(T_(s)) composition to form a compressible paste wherein theratio of water to M_(n+1)X_(n)(T_(s)) MXene is in a range of from about0.3 to about 0.65 by mass; and (b) compressing an amount of theM_(n+1)X_(n)(T_(s)) composition with a pressure of at least 5 psig toform a solid body; said M_(n+1)X_(n)(T_(s)) composition comprising atleast one layer having a first and second surface, each layer comprisinga substantially two-dimensional array of crystal cells; each crystalcell having an empirical formula of M_(n+1)X_(n), such that each X ispositioned within an octahedral array of M, wherein M is at least oneGroup 3 to 7 (corresponding to Group IIIB, IVB, VB, VIB, or VIB) metal,wherein each X is C, N, or a combination thereof and n=1, 2, or 3;wherein at least one of said surfaces of the layers has surfaceterminations comprising alkoxide, alkyl, carboxylate, halide, hydroxide,hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide, sulfonate,thiol, or a combination thereof; wherein said body is electricallyconductive and exhibits (i) a resistivity in a range of from about 1 toabout 10,000 micro-ohm-meters; (ii) volumetric capacity of at least 500F/cm³ when tested at a scan rate of 20 mV/s or at least 550, 600, 650,700, 750, or 800 F/cm³ when tested as a slower scan rate; or (iii) both(i) and (ii).

Embodiment 2

The method of Embodiment 1, wherein the compressing is done at apressure in a range of from about 5 psig to about 500 psig.

Embodiment 3

The method of Embodiment 1 or 2, wherein the compression is accomplishedat least in part by rolling or compression molding or otherwiseflattening or shaping, e.g., using heated or unheated platen orplaten-like device.

Embodiment 4

The method of any one of Embodiments 1 to 3, wherein the compressing isaccomplished at least in part by rolling the M_(n+1)X_(n)(T_(s))composition between at least two roller bars.

Embodiment 5

The method of any one of Embodiments 1 to 4, wherein the compressing ofthe M_(n+1)X_(n)(T_(s))composition is accompanied by the removal ofwater.

Embodiment 6

The method of any one of Embodiments 1 to 5, wherein the solid body iselectrically conductive.

Embodiment 7

The method of any one of Embodiments 1 to 6, wherein the solid body hassurface electrical resistivity in a range of from about 1micro-ohm-meters to about 10,000 micro-ohm-meters.

Embodiment 8

The method of any one of Embodiments 1 to 7, wherein the solid body iscompressed to a thickness in a range of from about 0.1 micron to about100 microns.

Embodiment 9

The method of any one of claims 1 to 8, wherein the M_(n+1)X_(n)(T_(s))composition comprises a plurality of M_(n+1)X_(n)(T_(s))flakes having atleast one mean lateral dimension in a range of from about 0.5 micron toabout 5 microns.

Embodiment 10

The method of any one of Embodiments 1 to 9, wherein M is at least oneGroup 4, 5, or 6 metal or Mn.

Embodiment 11

The method of any one of Embodiments 1 to 10, wherein M is at least oneof Hf, Cr, Mn, Mo, Nb, Sc, Ta, Ti, V, W, or Zr.

Embodiment 12

The method of any one of Embodiments 1 to 11, wherein M is Ti, and n is1 or 2.

Embodiment 13

The composition of any one of Embodiments 1 to 12, wherein M_(n+1)X_(n)comprises Sc₂C, Sc₂N, Ti₂C, Ti₂N, V₂C, V₂N, Cr₂C, Cr₂N, Zr₂C, Zr2N,Nb₂C, Nb₂N, Hf₂C, Hf₂N, Ta₂C, Mo₂C, Ti₃C₂, Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂,Mo₃C₂, (Cr_(2/3) Ti_(1/2))₃C₂, Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃, Ta₄N₃,Nb₄C₃, or a combination thereof.

Embodiment 14

The method of any one of Embodiments 1 to 13, the crystal cells havingan empirical formula Ti₃C₂ or Ti₂C and wherein at least one of saidsurfaces of each layer is coated with surface terminations, T_(s),comprising alkoxide, fluoride, hydroxide, oxide, sub-oxide, sulfonate,or a combination thereof.

Embodiment 15

The method of any one of Embodiments 1 to 14, wherein theM_(n+1)X_(n)(T_(s)) composition is formed by removing at least 90%, 95%,99%, or 99.9% of the A atoms from a MAX-phase composition having anempirical formula of M_(n+1)AX_(n).

-   wherein M is at least one Group 3, 4, 5, 6, or 7 metal,-   wherein A is an A-group (Group 13 or 14) element;-   each X is C, N, or a combination thereof; and-   n=1, 2, or 3.

Embodiment 16

The method of Embodiment 15, wherein the removal of the A atoms is donein aqueous media.

Embodiment 17

The method of Embodiments 15 or 16, wherein M is at least one of Hf, Cr,Mn, Mo, Nb, Sc, Ta, Ti, V, W, or Zr.

Embodiment 18

The method of any one of Embodiments 15 to 17, wherein A is at least oneof Al, As, Ga, Ge, In, P, Pb, S, or Sn.

Embodiment 19

The method of any one of Embodiments 15 to 18, wherein the A atoms areremoved by a process comprising a treatment with a fluorine-containingacid.

Embodiment 20

The method of Embodiment 19, wherein the fluorine-containing acid isaqueous hydrofluoric acid.

Embodiment 21

The method of Embodiment 19 wherein the fluorine-containing acidcomprises:

-   (a) aqueous ammonium hydrogen fluoride (NH₄F·HF);-   (b) an alkali metal bifluoride salt (i.e., QHF₂, where Q is Li, Na,    or K); or-   (c) a fluoride salt, for example an alkali metal, alkaline earth    metal, or ammonium fluoride salt (such as LiF, NaF, KF, CsF, CaF₂,    tetraalkyl ammonium fluoride (e.g., tetrabutyl ammonium fluoride))    in the presence of at least one mineral acid stronger than HF (such    as HCl, HBr, HI, H₃PO₄, HNO₃, oxalic acid, or H₂SO₄).

Embodiment 22

The method of Embodiment 21, wherein the fluorine-containing acid isderived from lithium fluoride and an aqueous mineral acid, such as HCl,HBr, HI, H₃PO₄, HNO₃, oxalic acid, or H₂SO₄, preferably HCl.

Embodiment 23

The method of any one of Embodiments 1 to 22, said method being capableof providing a solid body exhibiting a volumetric capacitance in a rangeof about 500 F/cm³ to about 1500 F/cm³, or in a range of from about 100F/g to about 500 F/g, when tested as a scan rate of 2 mV/s.

Embodiment 24

A solid body prepared by any one of Embodiments 1 to 23.

Embodiment 25

A solid body prepared by anyone of Embodiments 1 to 23, furthercomprising intercalated lithium or other metal ions (such as alkali oralkaline earth or transition metal ions).

Embodiment 26

The solid body of Embodiment 24 or 25, wherein the solid body exhibitinga volumetric capacitance of from about 500 F/cm³ to about 2500 F/cm³, orin a range of from about 200 F/g to about 500 F/g, when tested as a scanrate of 2 mV/s.

Embodiment 27

A solid body comprising or consisting essentially of aM_(n+1)X_(n)(T_(s)) material exhibiting a volumetric capacity greaterthan about 500 F/cm³, up to about 2500 F/cm³; said M_(n+1)X_(n)(T_(s))material comprising a composition comprising plurality of layers, eachlayer having a first and second surface, each layer comprising

-   a substantially two-dimensional array of crystal cells.-   each crystal cell having an empirical formula of M_(n+1)X_(n), such    that each X is positioned within an octahedral array of M,-   wherein M is at least one 3, 4, 5, 6, or 7 metal,-   wherein each X is C, N, or a combination thereof and-   n=1, 2, or 3;-   wherein at least one of said surfaces of the layers has surface    terminations comprising alkyl, alkoxide, carboxylate, halide,    hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide,    sulfonate, thiol, or a combination thereof

Embodiment 28

The solid body of Embodiment 27, the M_(n+1)X_(n)(T_(s)) MXene furthercomprising intercalated lithium ions, lithium atoms, or a combination oflithium ions and lithium atoms.

Embodiment 29

An electrode comprising a solid body of any one of Embodiments 24 to 28.

Embodiment 30

An electrochemical device comprising a solid body of any one ofEmbodiments 24 to 28 or an electrode of Embodiment 29.

Embodiment 31

A method comprising,

-   (a) applying a MXene dispersion onto a substrate surface, said MXene    dispersion comprising (or consisting essentially of) at least one    type of MXene platelets dispersed in a solvent; and-   (b) removing at least a portion of solvent so as to provide a coated    film of at least one layer of MXene platelets oriented to be    essentially coplanar with the substrate surface,-   said MXene platelets comprising (or consisting essentially of) a    M_(n+1)X_(n)(T_(s)) composition having at least one layer, each    layer having a first and second surface, each layer comprising-   a substantially two-dimensional array of crystal cells.-   each crystal cell having an empirical formula of M_(n+1)X_(n) such    that each X is positioned within an octahedral array of M,-   wherein M is at least one Group 3, 4, 5, 6, or 7,-   wherein each X is C, N, or a combination thereof and-   n=1, 2, or 3;-   wherein at least one of said surfaces of the layers has surface    terminations, T_(s), independently comprising alkoxide, alkyl,    carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,    sub-nitride, sulfide, sulfonate, thiol, or a combination thereof;-   said coated film being electrically conductive and exhibiting:    -   (i) a resistivity in a range of from about 0.01 to about 1000        micro-ohm-meters, preferably 1-10 micro-ohm-meters;    -   (ii) an ability to transmit at least about 50%, at least about        55%, at least about 60%, at least about 65%, at least about 70%,        at least about 75%, at least about 8%, at least about 85%, at        least about 90%, or at least about 95% of incident light of at        least one wavelength in a range of from about 300 nm to about        2000 nm, about 300 nm to about 800 nm, or about 500 nm to about        2500 nm ;    -   (iii) a ratio of DC conductivity, measured in Siemens/meter, to        light absorbance, (including visible light absorbance), measured        as a decadic absorbance per meter, of at least 0.1 Siemens        measured at at least one wavelength in the range of 300 to 2500        nm;    -   (iv) a value of the real dielectric permittivity less than zero        or less than negative one for wavelengths greater than a        threshold wavelength, for example, 500 nm; or    -   (v) a combination of any two or more of (i), (ii), (iii), and        (iv).

Embodiment 32

The method of Embodiment 31, wherein the MXene dispersion is applied tothe substrate surface by spin coating.

Embodiment 33

The method of Embodiment 31 or 32 wherein the MXene dispersion isapplied dropwise onto the an optionally rotating substrate surface,during or after which the substrate surface is made to rotate at a ratein a range of from about 300 rpm (rotations per minute) to about 5000rpm.

Embodiment 34

The method of Embodiment 31, wherein the MXene dispersion is applied tothe substrate surface by brushing, dipcoating, or doctor blading.

Embodiment 35

The method of Embodiment 31 or 34, wherein the MXene dispersion isapplied to the substrate surface by brushing or dipcoating, followed byrotating the substrate surface at a rate in a range of from about 300rpm to about 5000 rpm.

Embodiment 36

The method of any one of Embodiments 31 to 35, wherein the MXenedispersion is an aqueous dispersion optionally comprising one or moresurfactants.

Embodiment 37

The method of any one of Embodiments 31 to 36, wherein the MXenedispersion comprising an organic solvent, preferably a polar solventsuch as an alcohol solvent. Some sub-embodiments include those where thepolar solvent comprises ethanol, isopropanol, dimethylformaide,pyridine, dimethylsulfoxide, or a mixture thereof

Embodiment 38

The method of any one of Embodiments 31 to 37, wherein the substrate isrigid.

Embodiment 39

The method of any one of Embodiments 31 to 37, wherein the substrate isflexible.

Embodiment 40

The method of any one of Embodiments 31 to 39, wherein the coating isareal.

Embodiment 41

The method of any one of Embodiments 31 to 39, wherein the coating ispatterned on the substrate.

Embodiment 42

The method of any one of Embodiments 31 to 41, wherein at least aportion of solvent is removed by evaporation.

Embodiment 43

The method of any one of Embodiments 31 to 43, the coated filmcomprising an overlapping array of two or more overlapping layers ofMXene platelets oriented to be essentially coplanar with the substratesurface.

Embodiment 44

The method of any one of Embodiments 31 to 43, wherein the solid bodyhas surface electrical resistivity in a range of from about 1micro-ohm-meters to about 10 micro-ohm-meters, from about 10micro-ohm-meters to about 100 micro-ohm-meters, from about 100micro-ohm-meters to about 1000 micro-ohm-meters, from about 1000micro-ohm-meters to about 10,000 micro-ohm-meters, or any combination oftwo or more of these ranges.

Embodiment 45

The method of any one of Embodiments 31 to 44, wherein theM_(n+1)X_(n)(T_(s)) composition comprises a plurality ofM_(n+1)X_(n)(T_(s)) platelets having at least one mean lateral dimensionin a range of from about 0.1 micron to about 50 microns.

Embodiment 46

The method of any one of Embodiments 31 to 45, wherein M is at least oneGroup 4, 5, 6, or 7 metal.

Embodiment 47

The method of any one of Embodiments 31 to 46, wherein M is at least oneof Hf, Cr, Mn, Mo, Nb, Sc, Ta, Ti, V, W, or Zr.

Embodiment 48

The method of any one of Embodiments 31 to 47, wherein M is Ti, and n is1 or 2.

Embodiment 49

The method of any one of Embodiments 31 to 48, wherein M_(n+1)X_(n)comprises Sc₂C, Sc₂N, Ti₂C, Ti₂N, V₂C, V₂N, Cr₂C, Cr₂N, Zr2C, Zr2N,Nb₂C, Nb₂N, Hf₂C, Hf₂N, Ta₂C, Mo₂C, Ti₃C₂, Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂,Mo₃C₂, (Cr_(2/3) Ti_(1/2))₃C₂, Ti₄C₃, Ti₄N₃, V₄C₃, V₄N₃, Ta₄C₃, Ta₄N₃,Nb₄C₃, or a combination thereof

Embodiment 50

The method of any one of Embodiments 31 to 47, wherein M comprises atleast two Group 4, 5, 6, or 7 metals, and the M_(n+1)X_(n)(T_(s))composition is represented by a formula M′₂M″_(m)X_(m+1)(T_(s)), wherem=n−1.

Embodiment 51

The method of Embodiment 50, wherein M′ comprises Ti, V, Cr, or Mo.

Embodiment 52

The method of Embodiment 49 or 50, wherein M″ comprises Ti, V, Nb, orTa, and M′ is different than M″.

Embodiment 53

The method of any one of Embodiments 49 to 52, wherein M′₂M″_(m)X_(m+1),comprises Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂, Mo₂Ti₂C₃, Cr₂TiC₂, Cr₂VC₂,Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, V₂TiC₂, or a combinationthereof

Embodiment 54

The method of any one of Embodiments 49 to 52, wherein M′₂M″_(m)X_(m+1),comprises Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂, Mo₂NbC₂, Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂,Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, V₂TiC₂, or a combination thereof

Embodiment 55

The method of any one of Embodiments 49 to 52, wherein M′₂M″_(m)X_(m+1),comprises Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃, Cr₂Ti₂C₃, Cr₂V₂C₃,Cr₂Nb₂C₃, Cr₂Ta₂C₃, Nb₂Ta₂C₃, Ti₂Nb₂C₃, Ti₂Ta₂C₃, V₂Ta₂C₃, V₂Nb₂C₃,V₂Ti₂C₃, or a combination thereof

Embodiment 56

The method of any one of Embodiments 49 to 52, whereinM′₂M″_(m)X_(m+1)i, comprises Nb₂VC₂, Ta₂TiC₂, Ta₂VC₂, Nb₂TiC₂ or acombination thereof

Embodiment 57

The method of any one of Embodiments 49 to 56, wherein theM′₂M″_(m)X_(m+1) is in a disordered state.

Embodiment 58

The method of any one of Embodiments 31 to 57, the crystal cells havingan empirical formula Ti₃C₂ or Ti₂C and wherein at least one of saidsurfaces of each layer is coated with surface terminations, T_(s),comprising alkoxide, fluoride, hydroxide, oxide, sub-oxide, sulfonate,or a combination thereof

Embodiment 59

The method of any one of Embodiments 31 to 58, wherein theM_(n+1)X_(n)(T_(s)) or M′₂M″_(m)X_(m+1) composition is formed byremoving at least 90% the A atoms from a MAX-phase composition having anempirical formula of M_(n+1) AX_(n) or M′₂M″_(m)AX_(m+1), respectively;

-   wherein M is at least one Group 3, 4, 5, 6, or 7 metal,-   wherein A is an A-group element;-   each X is C, N, or a combination thereof; and-   n=1, 2, or 3.

Embodiment 60

The method of Embodiment 59, wherein the removing the A atoms is done inaqueous media.

Embodiment 61

The method of Embodiment 59 or 60, wherein M is at least one of Hf, Cr,Mn, Mo, Nb, Sc, Ta, Ti, V, W, or Zr

Embodiment 62

The method of any one of Embodiments 59 to 61, wherein A is at least oneof Al, As, Ga, Ge, In, P, Pb, S, or Sn.

Embodiment 63

The method of any one of Embodiments 59 to 62, wherein the A atoms areremoved by a process comprising a treatment with a fluorine-containingacid.

Embodiment 64

The method of Embodiment 63, wherein the fluorine-containing acid isaqueous hydrofluoric acid.

Embodiment 65

The method of Embodiments 63 wherein the fluorine-containing acidcomprises or is derived from:

-   (a) aqueous ammonium hydrogen fluoride (NH₄F·HF);-   (b) an alkali metal bifluoride salt (i.e., QHF₂, where Q is Li, Na,    or K), or a combination thereof; or-   (c) at least one fluoride salt, such as an alkali metal, alkaline    earth metal, or ammonium fluoride salt (e.g., LiF, NaF, KF, CsF,    CaF₂, tetraalkyl ammonium fluoride (e.g., tetrabutyl ammonium    fluoride)) in the presence of at least one mineral acid that is    stronger than HF (such as HCl, HNO₃, or H₂SO₄); or-   (d) a combination of two or more of (a)-(c).

Embodiment 66

The method of Embodiment 65, wherein the fluorine-containing acid isderived from lithium fluoride and an aqueous mineral acid that isstronger than HF, such as HCl, HNO₃, or H₂SO₄, preferably HCl.

Embodiment 67

A coating prepared by the method of any one of Embodiments 31 to 66.

Embodiment 68

A coating prepared by the method of any one of Embodiments 31 to 66,further comprising intercalated lithium ions, lithium atoms, sodiumions, sodium, or a combination thereof

Embodiment 69

An electrical device containing the coating of Embodiment 67 or 68.

Embodiment 70

The electrical device of Embodiment 69 that is a liquid crystal display,a light emitting diode (including organic light emitting diodes), atouchscreen, or a photovoltaic device.

Embodiment 71

The electrical device of Embodiment 69 that is a plasmonic device suchas a plasmonic sensor array, an optical-to-electrical signal transducera surface-plasmon polariton transmitter, an infrared-reflecting window.

Embodiment 72

A metamaterial such as a cloaking material, transformational opticcomponent, superlens comprising a coating prepared by the method of anyone of Embodiments 31 to 66.

EXAMPLES

The following Examples are provided to illustrate some of the conceptsdescribed within this disclosure. While each Example is considered toprovide specific individual embodiments of composition, methods ofpreparation and use, none of the Examples should be considered to limitthe more general embodiments described herein.

Example 5 General Remarks

M_(n+1)X_(n)(T_(s)) MXenes are a relatively young class of 2D solids,produced by the selective etching of the A-group layers from the MAXphases, a >70 member family of layered, hexagonal early transition metalcarbides and nitrides. To date, all M_(n+1)X_(n)(T_(s)) MXenes have beenproduced by etching MAX phases in concentrated hydrofluoric acid, HF, orammonium bifluoride. M_(n+1)X_(n)(T_(s)) MXenes have already proven tobe promising candidates for electrodes in Li-ion batteries andsupercapacitors, exhibiting volumetric capacitances exceeding mostpreviously reported materials. However, the path to electrodemanufacturing required the handling of concentrated HF and a laboriousmulti-step procedure. Herein a safer route was sought by exploiting thereaction between common, inexpensive hydrochloric acid, HCl, andfluoride salts. Furthermore, given M_(n+1)X_(n)(T_(s)) MXenes' abilityto preferentially intercalate cations (post-synthesis), a relatedquestion was whether etching and intercalation might be achieved in asingle step, as was observed for etching of thin Ti₃AlC₂ films withammonium bifluoride. Based on the change in M_(n+1)X_(n)(T_(s)) MXeneproperties upon intercalation and the compositional variability offluoride salts, this could lead to a one-step procedure for thesynthesis of many M_(n+1)X_(n)(T_(s)) MXenes, with tunable structuresand properties.

The M_(n+1)X_(n)(T_(s)) MXenes reported in this study were prepared bydissolving LiF in 6 M HCl, followed by the slow addition of Ti₃AlC₂powders and heating of the mixture at 40° C. for 45 h. After etching,the resulting sediments were washed to remove the reaction products andraise the pH (several cycles of water addition, centrifugation, anddecanting). This method also provided a good route for the intercalatingmetal ions into the MXene frameworks (e.g., see Example 8).

Example 1 Materials and Methods—Clays Example 1.1 Synthesis of Ti₃AlC₂

The MAX phase used as precursor for Ti₃C₂(T_(s)) synthesis herein,Ti₃AlC₂, was prepared by mixing commercial Ti₂AlC powders (Kanthal,Sweden) with TiC in a 1:1 molar ratio (after adjusting for the ˜12 wt. %Ti₃AlC₂ already present in the commercial powder), followed by ballmilling for 18 h. The mixture was then heated at 5° C./min, underflowing argon, Ar, in a tube furnace for 2 h at 1350° C. The resultinglightly sintered brick was ground with a TiN coated milling bit andsieved through a 400 mesh sieve producing powder with the particle sizeless than 38 μm.

Example 1.2 Synthesis of Ti₃C₂(T_(s)) MXene

Concentrated hydrochloric acid, HCl (Fisher, technical grade), was addedto distilled water to prepare a 6 M solution (30 mL total). 1.98 g (5molar equivalents) of LiF (Alfa Aesar, 98+%) was added to this solution.The mixture was stirred for 5 minutes with a magnetic Teflon stir bar todissolve the salt.

Three grams of Ti₃AlC₂ powders were carefully added over the course of10 minutes to avoid initial overheating of the solution as a result ofthe exothermic nature of the reactions. The reaction mixture was thenheld at 40° C. for 45 h, after which the mixture was washed through ˜5cycles of distilled water addition, centrifugation (3500 rpm×5 minutesfor each cycle), and decanting, until the supernatant reached a pH ofapproximately 6. The final product, with a small amount of water, wasfiltered on cellulose nitrate (0.22 μm pore size). At this stage, thefiltrate exhibited ‘clay-like’ properties and could be directlyprocessed into films by rolling.

Example 1.3 Preparation of Ti₃C₂(T_(s)) ‘Paper’

The Ti₃C₂(T_(s)) flakes were dispersed in distilled water (2 gTi₃C₂(T_(s)) per 0.5 L water), deaerated with Ar, followed by sonicationfor 1 h. The mixture was then centrifuged for 1 h at 3500 rpm, and thesupernatant, which was dark green in color, was collected. Thisdispersion was filtered using a membrane (3501 Coated PP, Celgard LLC,Charlotte, N.C.) to yield flexible, freestanding Ti₃C₂(T_(s)) paper. Theweight percentage of Ti₃C₂(T_(s)) delaminated into stable suspension inthis case was 45 wt %.

Example 1.4 Ti₃C₂(T_(s)) Clay Electrodes

Preparation of the Clay Electrodes is depicted step-by-step in FIGS.4A-4F. The dried and crushed Ti₃C₂(T_(s)) powder was hydrated to theconsistency of a thick paste, roughly 2 parts powder to 1 part water(a)-(c), which turns it into a plastic, ‘clay’-like state, that can beformed and molded. The ‘clay’ is then rolled using a roller mill withwater-permeable Celgard sheets (d) on either side, resulting in theformation of a freestanding film (e), which was readily lifted off ofthe membrane upon drying (f).

Example 2 Electrical Measurements Example 2.1 Activated Carbon (AC)Electrodes

The AC electrodes were prepared by mechanical processing of a pre-mixedslurry, containing ethanol (190 proof, Decon Laboratories, Inc.), YP-50activated carbon powder, (Kuraray, Japan), and polytetrafluoroethylene(PTFE) binder (60 wt. % in H₂O, Sigma Aldrich). The resulting ACelectrodes' composition was 95 wt. % AC and 5 wt. % PTFE. They hadthicknesses that varied between 100-150 μm; the mass densities per unitarea were in the 10-25 mg/cm² range.

Example 2.2 Electrochemical Setup

All electrochemical measurements were performed in 3-electrode Swagelokcells, where M_(n+1)X_(n)(T_(s)) MXene served as the working electrode,over-capacitive AC films were used as counter electrodes; Ag/AgCl in 1 MKCl was the reference electrode. Two layers of the Celgard membraneswere used as separators. The electrolyte was 1 M H₂SO₄ (Alfa Aesar, ACSgrade).

Example 2.3 Electrochemical Measurements

Cyclic voltammetry, CV, electrochemical impedance spectroscopy, EIS, andgalvanostatic cycling were performed using a VMP3 potentiostat(Biologic, France).

Cyclic voltammetry was performed using scan rates that ranged from 1 to100 mV/s. EIS was performed at OCP, with a 10 mV amplitude, andfrequencies that ranged from 10 mHz to 200 kHz.

Galvanostatic cycling was performed at 1 and 10 A/g between thepotential limits of −0.3 V to 0.25 V vs. Ag/AgCl. Capacitance datareported in the article were calculated from the slope of the dischargecurve.

Example 3 Characterization of Structure and Properties

XRD patterns were recorded with a powder diffractometer (RigakuSmartLab) using Cu Kα radiation (λ=1.54 Å) with 0.2° 2θ steps and 0.5 sdwelling time.

Scanning Electron Microscopy was performed on a SEM (Zeiss Supra 50 VP,Carl Zeiss SMT AG, Oberkochen, Germany) equipped with anEnergy-Dispersive Spectroscopy (EDS) (Oxford EDS, with INCA software).Most EDS scans were obtained at low magnification (100-200×) at randompoints of powdered samples. Elemental standards were as follows: C:CaCO₃; Al:Al₂O₃; O:SiO₂; F:MgF₂; Nb:Nb metal. XPP matrix correction(Pouchou and Pichoir, 1988) was used for elemental quantitativeanalysis.

Transmission electron microscopy of the Ti₃C₂(T_(s)) flakes wasperformed on a TEM (JEOL JEM-2100, Japan) using an accelerating voltageof 200 kV. The TEM samples were prepared by dropping two drops ofdiluted colloidal solution of Ti₃C₂(T_(s)) flakes onto a copper grid anddried in air. The flake size and number of layers per flakedistributions were obtained through statistical analysis of more than300 Ti₃C₂(T_(s)) flakes in the TEM images.

Resistivity measurements were performed with a 4-point probe (ResTestv1, Jandel Engineering Ltd., Bedfordshire, UK). Measured resistivity wasautomatically multiplied by the proper thickness correction factor givenby the Jandel software.

Example 4 Additional Experiments

Effect of Synthetic Conditions: Experiments showed that the reactionconditions of 35° C. for 24 h rather than 40° C. for 45 h produced amaterial with persistent MAX peaks in XRD, and higher Al content by EDS,but that gave reliable high yields of delaminated flakes uponsonication. The Ti₃AlC₂ etched at higher temperatures showed lower Alcontent but did not always readily delaminate and disperse bysonication.

Detailed volumetric analysis: In order to quantify diffusion-limitedcontributions to capacitance, the relationship between where the currenti(V) (at a given voltage, V) in mA and scan rate, υ, in V/s, was assumeto be:i(V)=k ₁ υ+k ₂υ^(0.5),where k₁ and k₂ are constants. For the CVs, at scan rates from 1 mV/s to20 mV/s, current values were extracted, and i/υ⁰⁵ vs. υ^(0.5) wasplotted at each voltage and linear fitting was performed:i(V)/υ^(0.5)=k₁υ^(0.5)+k₂. The slope k1, for each voltage, describes thecontributions of the non-diffusion controlled processes to the overallprocess.

The resulting sediment formed a clay-like paste that could be rolled,when wet (FIG. 1A), between water-permeable membranes in a roller millto produce flexible, free-standing films (FIG. 1C) in a matter ofminutes comparable to those produced by the laborious technique ofintercalation, delamination, and filtration in the past. A graphicaldepiction of the processing is provided in FIGS. 4A-4F. Further, scalingwas not limited to the size of the filtration apparatus; films of anydimensions could be readily produced. Additionally, when wet, the ‘clay’could be molded and dried to yield various shapes that were highlyconductive (FIG. 1D). Diluted, it could also be used as an ink todeposit (print) M_(n+1)X_(n)(T_(s)) MXene on various substrates. Likeclay, the material could be rehydrated, swelling in volume, andshrinking when dried (FIG. 1B).

Energy-dispersive spectroscopy (EDX) confirmed that aluminum, Al, wasremoved, and X-ray diffraction (XRD) showed the disappearance of Ti₃AlC₂peaks (traces can be seen in the case of incomplete transformation).Multilayer particles did not show the accordion-like morphology seen inHF-etched M_(n+1)X_(n)(T_(s)) materials reported to date; rather,particles appeared tightly stacked, presumably as a result of waterand/or cationic intercalation (see FIG. 5A). Fluorine and oxygen wereobserved in EDX; this, coupled with XPS showing evidence of Ti—F andTi—O bonding, suggests O- and F-containing surface terminations, as hasbeen discussed at length for HF-produced M_(n+1)X_(n)(T_(s)) materials.

XRD patterns of the etched material, in its air-dried multilayeredstate, showed a remarkable increase in the intensity and sharpness ofthe (0001 ) peaks (FIG. 2A, bottom trace); in some cases the full widthat half maximum (FWHM) was as small as 0.188°, as opposed to the broadpeaks typical of HF-etched M_(n+1)X_(n)(T_(s)) MXene, and more typicalof intercalated M_(n+1)X_(n)(T_(s)) MXenes. Further, compared to a clattice parameter of ca 20 Å for HF-produced Ti₃C₂(T_(s)), thecorresponding value herein was 27-28 Å. XRD patterns of still-hydratedsediment showed shifts to even higher spacings—c lattice parameters ashigh as ca. 40 Å have been measured. These large shifts are suggestiveof the presence of water, and possibly cations, between the hydrophilicand negatively charged M_(n+1)X_(n)(T_(s)) MXene sheets. Based on thesesubstantial increases in c lattice parameters and the clay-likeproperties (see below), it is reasonable to assume that—like inclays—the swelling is due to the intercalation of multiple layers ofwater and possibly cations between the M_(n+1)X_(n)(T_(s)) MXene sheets.Interfacial water has a more structured hydrogen bonding network thanbulk H₂O. The M_(n+1)X_(n)(T_(s)) MXene surface, holding a negativeelectric charge, may act to align the dipoles of water molecules betweenM_(n+1)X_(n)(T_(s)) MXene layers.

When the ‘clay’ was rolled into freestanding films, XRD patterns showedagain strong ordering in the c direction (FIG. 2A, upper trace). Films,ranging in thicknesses from sub-micron to ca. 100 μm, were readilyproduced by this method. The most compelling evidence for particleshearing is the significant intensity decrease of the (110) peak around61°, indicating a reduction of ordering in the non-basal directionswhile order in the c-direction was maintained (see upper XRD pattern inFIG. 2A and SEM in FIG. 5B). Morphologically, the thinner films showedmore overall shearing of the multilayer particles when viewed in crosssection (FIGS. 2E and F) and exhibited substantial flexibility, evenwhen allowed to dry thoroughly (inset). Attempts to hydrate and rollHF-produced M_(n+1)X_(n)(T_(s)) MXene were unsuccessful; it is thoughtat this time that the intercalated water—believed to be absent with HFalone is used as an ethcant—acts as a lubricant that allows facileshearing.

The c parameter expansion also resulted in the weakening of interactionsbetween the M_(n+1)X_(n)(T_(s)) MXene layers as evidenced by easydelamination of multilayered particles by sonication, like it is donefor van der Waals solids. In our previous work, typical sonication timesfor delamination (after post-synthesis intercalation with dimethylsulfoxide) were of the order of 4 h. Here, sonication times of the orderof 30-60 minutes resulted in stable suspensions with concentrations ashigh as 2 g/L, higher than observed previously. Remarkably, the yieldfrom multilayer to dispersed flakes was about 45% by mass. Freestandingfilms were also readily fabricated by filtering these suspensions, asreported previously.

The fact that the LiF—HCl etchant was much milder than HF resulted inflakes with larger lateral dimensions (FIG. 2B) that did not containnanometer-size defects frequently observed in HF-etched samples. TEManalysis showed that, of 321 flakes analyzed, over 70% had dimensions of0.5-1.5 μm (FIGS. 6A-B). Single, ca. 10 Å thick layers were imaged byTEM (FIGS. 2C, D), confirming that the material is indeedtwo-dimensional. Analysis of 332 flakes suggested that roughly 70% ofthe flakes were 1-2 layers thick (FIGS. 6C-F). We note that, since therestacking or folding of flakes can lead to higher apparent thicknesses(FIG. 7), the 70% estimate is conservative. Thus, using this method,large fractions of single-layered M_(n+1)X_(n)(T_(s)) flakes with highyields, large lateral sizes, and good quality can be readily produced.

Previously Ti₃C₂(T_(s)) “paper”—made by filtration of solutionscontaining delaminated Ti₃C₂(T_(s)) flakes—has been shown to exhibitvolumetric capacitances of ca. 350 F/cm³ at 20 mV/s (and 450 F/cm³ at 2mV/s) in KOH electrolyte. For comparison, herein we characterized theelectrochemical performances of rolled, freestanding Ti₃C₂(T_(s)) filmsin 1 M sulfuric acid, H₂SO₄. Advantages of acidic electrolytes includenot only their excellent conductivities but also that protons, being thesmallest cations, are known to allow for surface redox reactions intransition metal oxide electrodes, such as RuO₂, MnO₂ and some others,and may provide additional contribution to capacitance from fast surfaceredox.

At a scan rate of 2 mV/s, capacitance values reached 900 F/cm³ (FIG. 3A)and a good rate handling ability was observed (FIG. 3B). Highercapacitances are believed possible with optimization. Theresults—summarized and compared with previous work (comparison marked as[8]; see Lukatskaya, M. R. et al. Cation intercalation and highvolumetric capacitance of two-dimensional titanium carbide. Science 341,1502-1505 (2013)) in FIG. 3B—clearly show that rolled Ti₃C₂(T_(s)) clayelectrodes show outstanding capacitive performance, not onlyvolumetrically but gravimetrically as well, achieving 245 F/g at 2 mV/s.This can be ascribed to the smaller size of H⁺ compared to otherintercalating cations, surface redox processes, and improvedaccessibility of interlayer spacing in LiF—HCl etchedM_(n+1)X_(n)(T_(s)) MXene due to pre-intercalated water, compared to thepreviously studied HF-etched samples. The electrodes showed nomeasurable capacitance losses even after 10,000 cycles (FIG. 3C).Coulombic efficiency is close to 100% (inset in FIG. 3C), confirmingthat the outstanding performance is not due to parasitic reactions.

To quantify the capacitive and diffusion limited contributions to thetotal capacitances, Dunn et al's approach was used. See, Wang, J.,Polleux, J., Lim, J. & Dunn, B. , J. Phys. Chem. C 111, 14925-14931(2007). The results of this analysis—summarized in FIG. 3D—show that, atscan rates below 20 mV/s, there is a noticeable, yet not prevailing,contribution of diffusion-limited processes to the total capacitance; atscan rates of 20 mV/s and higher, the response is notdiffusion-controlled but is rather due to surface capacitive effects,whether electrostatic or pseudocapacitive. Further, if there are alsoredox contributions from changes in the oxidation states of surface Tiatoms layers, the redox processes are not diffusion-limited, and thusrepresent “intrinsic” capacitive behavior.

When the electrochemical responses of three rolled clay electrodes—5μm,30 μm and 75 μm thick—were compared (FIGS. 3E, F), not surprisingly, thevolumetric capacitances decreased with increased thickness. Thesethickness-dependent differences can be partially traced to the electrodemorphologies. As noted above, electrodes thinner than 10 μm showed goodflake alignment (FIG. 2E) with typical densities of 3.6-3.8 g/cm³. At2.2-2.8 g/cm³, the densities of the thicker (15 μm and larger) rolledelectrodes were lower, which is a reflection of the fact that their coreseemed to be more open (FIG. 2F). And while the lower densities led tolower volumetric capacitances, their more open structure ensuredaccessibility to ions and thus similar rate performances as theirthinner counterparts (FIGS. 3E, F). The lower densities also ensuredthat the drop in gravimetric capacitances with thickness (see FIG. 8D)was not that significant.

The good capacitive rate performance of the 75 μm thick electrodes (FIG.3E) is noteworthy, however, and demonstrates scalability and hugepromise of M_(n+1)X_(n)(T_(s)) MXenes for application as negativeelectrodes of hybrid large scale energy storage devices. Electrodes ofthat thickness cannot be produced by filtration and theM_(n+1)X_(n)(T_(s)) MXene clay-like characteristics add importantversatility to electrode manufacturing, allowing films of the requiredthicknesses to be rolled. Note that the capacitance values reportedherein are still preliminary. As better understanding of how the films'morphologies affect their capacitances is gained, significantenhancements in the latter should ensue.

In terms of versatility, the LiF—HCl solution was also capable ofetching other MAX phases, e.g., Nb₂AlC and Ti₂AlC. In the case ofTi₂AlC, the multilayer powders were delaminated in a similar fashion toTi₃C₂(T_(s)) to produce suspensions of Ti₂C(T_(s)) flakes, as well asTi₂C(T_(s)) ‘paper’, which had not been previously reported. Theseconsiderations hint at the potential of this new etching method for thesynthesis of other M_(n+1)X_(n)(T_(s)) materials, which will be exploredin future studies.

This method of M_(n+1)X_(n)(T_(s)) production was successful to varyingdegrees for other fluoride salts, such as NaF, KF, CsF,tetrabutylammonium fluoride, and CaF₂ in HCl, all of which showedsimilar etching behavior. When H₂SO₄ was used instead of HCl,M_(n+1)X_(n)(T_(s)) materials were still obtained. We note here thatthese systems are options and merit further study; the ability to finetune the reaction based on reagents used will indubitably lead topotentially useful variations in compositions and properties, especiallysince it is reasonable to assume that different acids and salts shouldmodify the surface chemistries and pre-intercalate different ions.

In summary, a new high-yield method for M_(n+1)X_(n)(T_(s)) MXenesynthesis that is safer, easier, and provides a faster route todelaminated flakes has been detailed. This method yields a clay-likematerial, which can be shaped to give conductive solids of desiredforms, or rolled into thin sheets, for a host of applications. When therolled films were used as supercapacitor electrodes in a H₂SO₄electrolyte, the performances were extraordinary, with volumetriccapacitances up to 900 F/cm³ or 245 F/g. When it is further appreciatedthat these numbers are “first-generation” numbers that will no doubtincrease as we better understand the underlying processes and modify thematerial structure and chemistry, the potential of these non-oxide 2Dmaterials to push electrochemical energy storage to new heights isclear.

Example 6 Additional Work Example 6.1 Ti₃C₂(T_(s)) ‘Clay’

The ‘clay’ was produced by reacting Ti₃AlC₂ with a mixture ofhydrochloric acid (6 Molar concentration, in a ratio of 10 mL acid:1 gTi₃AlC₂) and lithium fluoride (5 molar equivalents per equivalentTi₃AlC₂) at 40° C. for 24 h. Washing the resulting sediment withdistilled water to a pH of 6-7, followed by collection via suctionfiltration, yielded a hard ‘clay’-like solid upon drying; this solid wascrushed to yield a fine powder that could be readily rehydrated.

Example 6.2 Rolled Films

The powder from Example 6.1 was hydrated fully by immersing in water ina ratio of 0.1 g Ti₃C₂(T_(s)):10 g water for 10 minutes with agitation,followed by suction filtration to yield hydrated ‘clay’ ready forprocessing. The ‘clay’ was taken directly and sandwiched in between twopieces of Celgard (3501 coated PP, Celgard LLC, Charlotte, N.C.)membrane, followed by insertion into a roller mill set to desiredthickness; rolling followed by drying yielded a detachable film (upondrying, the resulting thickness could be made to range from sub-micronto over 100 μm.

Example 7 SpinCast (Spin Coated) Angstrom-Thin Conductors

MAX etching and washing. 2.5 mL of deionized water, 2.5 mL of 12 Mhydrochloric acid, 0.333 g of lithium fluoride (97%, Acros Organics),and 0.5 g of MAX-phased Ti₃AlC₂, produced by a previously describedmethod (M. Ghidiu, et al. “Conductive two-dimensional titanium carbide‘clay’ with high volumetric capacitance,” Nature, 516 (2014) 78-81),were combined in a plastic reaction vessel with a magnetic stir bar. Thereaction vessel was sealed and nitrogen gas was used to purge theheadspace for 20 minutes. After purging, the vessel was kept underpositive nitrogen pressure and heated to 40° C. in a silicon oil bath.The reaction was allowed to continue for 24 hours at 40° C. withstirring. The etched MXene was then transferred to a centrifuge tube andcombined with 40 mL of deionized water. The solution was thencentrifuged at 4180 RCF for 5 minutes. The supernatant was decanted towaste. This washing step was repeated four more times to removeremaining acid. After washing, the multilayer MXene was filtered andallowed to dry overnight.

Example 3 MXene Delamination

Deionized water was bubbled with nitrogen with vigorous stirring for 30minutes to purge oxygen. Dry multilayer MXene was combined with thiswater to a concentration of 100 mg/mL in a centrifuge tube. Thecentrifuge tube was then purged with nitrogen for 2 minutes to removeoxygen before sealing. The MXene solution was then sonicated in a bathsonicator for 1 hour to separate the individual MXene flakes from thestacks of multilayer MXene. After sonication, the solution wascentrifuged for 5 minutes at 4180 RCF to separate single flake MXenefrom multilayer MXene. The resulting supernatant was decanted to a newcentrifuge tube, while the pellet was discarded.

Example 4 MXene Spin Coating

The resulting supernatant was then used as an ink from which films couldbe spincoated. Films spincoated from this solution had transmittances ofless than 10%. This ink could be diluted to create thinner inks fromwhich more transparent films could be fabricated. Using a cleansubstrate, typical films were spincoated onto 1 in² substrates using 0.2mL of MXene ink at 1000 RPM for 60 seconds, followed by 2000 RPM for 5seconds to aid drying. The resulting transparent conductive films werestored under dry nitrogen overnight in order to fully dry any residualwater.

Example 5 Discussion of Results

Two-dimensional flakes of Ti₃C₂, known as MXene, were prepared based onthe method of Ghidiu providing a dispersion of MXene in water. Thisliquid starting material was used to fabricate transparent, conductivethin films on arbitrary substrates by spin coating. In a typicalfabrication, 0.2 mL of MXene solution was deposited onto a 1″ squarepiece of soda-lime glass at 1000 RPM for 1 minute at room temperature inopen atmosphere. Other substrates demonstrated include silicon waferswith a thermally grown SiO₂ layer, fused quartz rounds, and flexible,polyetherimide polymer films. By varying the concentration of the MXenein solution, the thickness of the resulting spincoated films can becontrolled. Absorbance spectra of films of varying thickness werecollected over a range of 200-3000 nm (FIG. 9A), from which the averagetransmittance between 550 nm and 1100 nm was calculated. The decline inthe transmissivity of visible light with increasing MXene concentrationcan be seen by eye, as in the photo inset in FIG. 9B. After drying in adry nitrogen environment overnight, a 4-point probe was used to measurethe sheet resistance of the spincoated films. Three regimes could beseen in the sheet resistance vs transmittance curve (FIG. 9B). The sheetresistance of films with transmittance between 2% at 85% had anexponential dependence on transmittance. Above 85%, the sheet resistanceincreased in a manner consistent with nearing the percolation threshold.Below 2% films transmittance was no longer a reliable measure of filmthickness, and sheet resistance dropped below the trend line.

SEM and AFM were used to characterize the thickness and the surfaceroughness of the spincoated MXene films. A sample of MXene on a siliconwafer with a thermally-grown oxide (Si/SiO₂) was cleaved and SEM used tocharacterize the film cross-section (FIGS. 10A and 10B). AFM was alsoused to measure the thickness of spincoated MXene films. By scratchingthe surface of the MXene film without damaging the SiO₂ layer beneath,the step height between the surface of the MXene film and substrate wasmeasured. SEM micrographs show the surface of these spincoated MXenefilms are primarily single flake or low-stack MXene with few filmdefects. AFM measures a surface roughness of 8.7 nm (FIG. 10C, inset).By comparing the thickness of films spincoated on Si/SiO₂ to thetransmittance of films spincoated on quartz under the same conditions,the absorption coefficient of spincoated MXene films was calculated tobe α=1.42×10⁵ cm⁻¹ (FIG. 10C).

Using the absorption coefficient determined from the fit of FIG. 10C,the transmittance data that provides the X-axis of FIG. 9B wastransformed into thickness in the range encompassing 2% to 85%transmittance. This was plotted against the sheet conductivity (FIG.11). A linear fit to this data provides a measure of the conductivity ofthe spincoated MXene films of σ=6.8×10³ S/cm. Because film thickness wascalculated from transmittance, light scattering present in even thethinnest films caused the offset seen in the data above.

Spectroscopic ellipsometry was used to provide an independent measure ofthe conductivity of these spincoated MXene films (FIGS. 12A and 12B).The complex reflectivity as a function of wavelength of MXene filmsspincoated on Si/SiO₂ substrates was collected at 50°, 60°, and 70°. Theoptical properties in the near-infrared were accurately modeled using aDrude oscillator with a resistivity of 1.36×10⁻⁴ Ω·cm, or equivalently aconductivity of 7.35×10³ S/cm. The uniqueness of this fit parameter wasdetermined by evaluating the goodness-of-fit for a range of values ofresistivity, while optimizing the other model parameters; for a 50%variation in resistivity, the mean-square error was observed to increaseby a factor of 6.

The stability of spincoated MXene films was also studied. By trackingthe film sheet resistance over time for films stored in open air orunder dry nitrogen, it could be seen that the stability of filmelectrical properties depend on both storage method and film thickness(FIG. 13A). Films stored under dry nitrogen displayed an increase insheet resistance of 14±5% over 11 weeks. In contrast, the sheetresistance of films stored in open air increased more quickly, occurringmore drastically in thinner films. This data is consistent with thediffusion of a species that degrades the electronic properties of thefilm. MXene are known to be subject to water intercalation¹. In order totest the effect of humidity on spincoated MXene films, the sheetresistance of MXene films was measured in a sequence of wetting anddrying (FIG. 13A). Resistance was measured for MXene films immediatelyafter spincoating from water, after overnight storage under drynitrogen, after storage for 2 days under nitrogen with saturated watervapor and finally, after a second overnight storage under dry nitrogen.Sheet resistances of films after a single wet-dry cycle were identicalto after a second wet-dry cycle with over a half order of magnitudeincrease in the sheet resistance during the wet stage. The sheetresistance of films stored in open air could be reduced by annealing indry nitrogen.

MXene dispersions can be deposited on a variety of substrates, includingflexible polymer substrates such as polyetherimide (FIG. 13A). Films onflexible substrates have conductivities similar to those spincoated onglass and retain this conductivity even at small radii of curvature(FIG. 14C). This retention of conductivity while flexing can beattributed to the 2D nature of MXene and sheets ability to slide overone another, allowing the film to flex without cracking. XRD confirmsthe presence of several layers of MXene (FIG. 14B) in spincoated films,while dropcast MXene ink reveals the presence of small amounts ofstacked MXene. This indicates that spincoating MXene ink eitherseparates the multilayer MXene from the lone flakes by throwing off thelarger MXene stacks during deposition or causes the multilayer MXene toshear into several-layer flakes.

MXene layers can be patterned by masking the selected areas of the MXenefilms while treating the unmasked or exposed portions to an etchant,such as nitric acid. The mask can be provided by, for example, adhesivetape or photoresist, in addition to other protective layers that wouldbe evident to someone skilled in the art. FIG. 15 shows a patternedMXene film, masked by adhesive tape strips after developing the exposedregions in nitric acid. The exposed MXene is oxidized to transparency.The tape is subsequently removed and the masked portion retains itsoriginal conductivity.

Example 8 Ion Exchange and Solvation Reactions in 2D Ti₃C₂ MXene

Protocols as also available for selective ion intercalation/exchange inMXene materials, as exemplified here with Ti₃C₂. The resulting materialshave different structural behaviors in response to water, and theexchange procedures are broadly applicable to many ions.

Experimental

Ti₃AlC₂: Ti₂AlC powders (−325 mesh, Kanthal, Sweden) were mixed with TiC(Alfa Aesar, 99.5% purity) and heated to 1200° C. (at a heating rate of10° C./min following by a 2 h soak) to afford Ti₃AlC₂, according topreviously-reported procedures.^([1)] The resulting solid was milledwith a milling bit and sieved (−400 mesh) to afford powders under 38 μmin size.

Ti₃C₂T_(x)-1: Ti₃AlC₂ powder (sieved to <38 μm particle size) was slowlyadded to 10 wt % hydrofluoric acid (HF) in a ratio of 1 g Ti₃AlC₂:10 mLetching solution. The reaction mixture was stirred for 24 h at 25° C.,after which the powders were washed with distilled water in acentrifugation and decantation process: water was added to the reactionmixture, it was shaken for 1 min, then centrifuged for 2 mins to collectthe powders. The supernatant was then discarded, and the processrepeated. This was done in a ratio of ˜0.5 g powders:40 mL water. Uponreaching a pH of ˜5, the powders were filtered to remove excess waterand left for another 24 h to dry in air.

Ti₃C₂T_(x)-2: Similar to the above procedure, Ti₃AlC₂ powder was addedto an etching mixture in the same ratio. In this case, however, theetchant was a mixture of 10% HF and LiCl. The etchant contained LiCl ina molar ratio 5 LiCl:1 Ti₃AlC₂. The mixture was stirred for 24 h at 25°C. followed by washing as described previously.

Acid pre-washing. To remove traces of LiF precipitated during etching,Ti₃C₂-2 was washed with a centrifugation procedure as described above,with three washes consisting of 6 M HCl (Fisher TraceMetal grade). Thisprocedure was performed directly after the etching of Ti₃C₂-2, beforeany of the sediments were allowed to dry.

Intercalation/exchange. For Ti₃C₂T_(x)-1, no prior acid washing wasperformed. For Ti₃C₂T_(x) -2, all samples were acid pre-washed asdescribed above. Before the samples were allowed to dry, salt solutions(1 N LiCl, NaCl, KCl, KBr, RbCl, MgCl₂, or CaCl₂ in distilled water)were added in a ratio of roughly 0.5 g MXene to 40 mL solution. Aftershaking for 2 min, the mixtures were allowed to sit for 1 h. The sampleswere then centrifuged to settle the powders, and the supernatants weredecanted and replaced with fresh solutions. The samples were againshaken and allowed to sit for 24 h. Then they were centrifuged, thesupernatants were discarded, and water was added, followed by agitationand centrifugation. After decanting, the sediment was collected viafiltration, and washed with distilled water (2×5 mL) followed by dryingin air (roughly 50% relative humidity) to yield desired powders.

Observations. Ti₃C₂T_(x) was prepared by the reaction of Ti₃AlC₂ with10% HF. After etching, removal of by-products by washing with water, anddrying, attempts were made to intercalate Li ions by immersion in 1 Maqueous LiCl. Even after 72 h of exposure, no major changes (not shown)were observed by x-ray diffraction (XRD). However, when LiCl was presentas part of the etchant (5 molar equivalents per mole of Ti₃AlC₂) ratherthan as a later addition, an intense and sharp (0002) reflection,corresponding to a c-LP of 24.5 Å, was observed for the powder dried inambient air (˜50% relative humidity for 24 h), as opposed to the broaderand less intense reflections of 19-20 Å often observed when only HF wasused. This material is designated Ti₃C₂-2. Some LiF was identified inXRD patterns of Ti₃C₂-2, most likely formed by precipitation. To removethis impurity, 6 M hydrochloric acid (HCl) was used to dissolve LiFduring the washing procedure following etching. Surprisingly, this stepresulted in the loss of order as observed by XRD (FIG. 16) with aconcurrent decrease of the c-LP to ˜21 A. However, washing with LiClafter this step was enough to restore the structure to that observedinitially, less the LiF impurity. It may be speculated that there is aproton/cation exchange, but more work is needed to confirm this. Thismaterial was designated Li—Ti₃C₂T_(x). It is not clear at this time whyTi₃C₂-1 does not behave similarly with LiCl treatment.

To study the evolution of the c-LPs upon drying, air-dried Li—Ti₃C₂T_(x)samples were prepared and saturated with distilled water immediatelyprior to measurement. The powders were then held in an atmosphere of˜40% relative humidity; FIG. 17A shows XRD patterns, recorded every 4min over ˜1 h of drying. From these results it is clear that only twophases were present: a fully hydrated phase with c-LP˜33 Å, and a phasewith c-LP 25 Å after drying. The lack of a continuously-shiftingintermediate peak signified an abrupt phase transition (FIG. 17C). Thetime dependence is shown as a contour plot (FIG. 17B) and by tracing themaximum intensity of each (0002) peak (FIG. 17D). Clearly at pointsduring drying, both phases were present simultaneously. Betweencompletion of the shift (roughly 1 h) and 24 h, the c-LP furtherdecreased from 25.0 Å to 24.5 Å. This change may be due to the removalof residual H₂O.

For Ti₃C₂-1, the c-LP (˜19.5 Å) from etching in HF alone was taken to bewithout H₂O intercalation. It followed that Li—Ti₃C₂T_(x) equilibratedat ˜40% relative humidity (c-LP of 24.5 Å) has a Δc of +5 Å, or a changeof +2.5 Å per interlayer space. As discussed in the literature, thischange corresponds roughly to the size of an H₂O molecule. Upon fullhydration, the single reflection corresponding to 33 Å (an interlayerexpansion of +6.8 Å) likely involved a bilayer of H₂O between the MXenelayers. This expansion matches well with that reported for othermaterials upon Li⁺ intercalation, and expansion of the 25-Å phasecorresponds to that of MoS₂ assumed to be intercalated with one layer ofH₂O. It was reasonable to assume that Li—Ti₃C₂T_(x) contained amonolayer of H₂O and ions, possibly in a crystallographic layer asobserved in materials such as layered double hydroxides. This 2-stageresponse highlighted in FIGS. 17A-D may be explained by postulating thatas H₂O leaves the structure, cation-H₂O clusters stabilize the bilayerstructure creating defects, and when a critical amount of H₂O is lost,the bilayer structure is no longer stable and collapses, leading to thestepwise transition observed.

To explore the intercalation of other ions, Ti₃C₂-2 was immersed in 1 Msolutions of NaCl, KCl, and RbCl, and 0.5 M solutions of MgCl₂ and CaCl₂following HCl treatment as described earlier, followed by washing withdistilled water to remove traces of salt, and drying in air to yieldsamples Na—Ti₃C₂T_(x), K—Ti₃C₂T_(x), Rb—Ti₃C₂T_(x), Mg—Ti₃C₂T_(x), andCa—Ti₃C₂T_(x). XRD patterns were recorded under full hydration (in thepresence of liquid water; FIG. 18A), after drying for 48 h in 40%humidity (FIG. 18B), and after 48 h of drying in the presence of P₂O₅(˜0% relative humidity; FIG. 18C). In all cases, XRD patterns showed thepresence of a single phase.

To test a hypothesis that the hydration of these materials was relatedto the enthalpy of hydration of the intercalated cations, the interlayerspace (viz. Δc/2) were plotted against e/r (where e is the cation chargeand r its radius), with radii values taken from the literature. Whensaturated with water (FIG. 18D), a clear trend emerged: changes ofroughly +3 Å or +6 Å per interlayer, which most likely correspond tomonolayers or bilayers of H₂O. Also plotted in FIG. 18D were theinterlayer space changes reported by Lerf and Schollhorn forcation-intercalated TiS₂ phases. See A. Lerf and R. Schollhorn, Inorg.Chem., 1977, 16, 2950-2956. The match was excellent, with very similarbasal expansions and a similar trend. It was clear that the cationtreatments led to different behaviours which, in the case of Ti₃C₂T_(x),seem to be described by the cation hydration enthalpy.

After drying in air, large changes were observed for Li—Ti₃C₂T_(x) andNa—Ti₃C₂T_(x) as their interlayer space shrunk to that of a monolayerspacing (FIG. 18E). Ca—Ti₃C₂T_(x) and Mg—Ti₃C₂T_(x) remained in thebilayer phase, likely due to their substantially higher hydrationenthalpies (Table 1).

TABLE 1 Experimental hydration enthalpies for selected cations CationHydration Enthalpy-ΔH°_(hyd) (kJ/mol)^([2]) Li⁺ 519 Na⁺ 409 K⁺ 322 Rb⁺293 Mg²⁺ 1921 Ca²⁺ 1577However, after drying over P₂O₅, Ca—Ti₃C₂T_(x) shrunk to the monolayerphase, while Mg—Ti₃C₂T_(x) displayed a broad peak centred on the bilayerregion (FIGS. 18C and 18F). FIG. 18G shows cartoons of the interlayerspace to explain these data. Full data are provided in Table 2. Thereason for the slight increase in c-LP after P₂O₅ for the alkali cationsis unclear at this time.

TABLE 2 Observed c parameters (d₍₀₀₀₂₎) for ion-intercalated samples(data plotted in main text FIG. 18) Ionic radius Charge/radius Hydratedc- 50% RH 0% RH (P₂O₅) Sample (Å)^([3]) Charge (e) (+e/Å) LP (Å) c-LP(Å) c-LP (Å) Li-Ti₃C₂T_(x) 0.94 +1 1.06 32.66 24.56 24.60 Na-Ti₃C₂T_(x)1.17 +1 0.85 31.24 24.76 24.91 K-Ti₃C₂T_(x) 1.49 +1 0.67 25.36 25.0825.14 Rb-Ti₃C₂T_(x) 1.63 +1 0.61 25.38 25.16 25.19 Mg-Ti₃C₂T_(x) 0.72 +22.78 31.11 29.83 30.09 Ca-Ti₃C₂T_(x) 1.00 +2 2.00 31.33 30.47 24.66

To further substantiate ion intercalation, x-ray photoelectronspectroscopy (XPS) was used. For Ti₃C₂-1, no Li-related peaks werepresent, either before or after sputtering (FIG. 19Ai. respectively).However, for Li—Ti₃C₂T_(x), the spectra before sputtering show thepresence of two peaks, one corresponding to a LiF and/or LiCl speciesand one corresponding to Li—O and/or Li—OH species (FIG. 19Aii). TheLi—O/Li—OH peak is at a binding energy (BE) of 54.2 eV. This speciesprobably originated from the presence of Li⁺ ions interacting with H₂Oor with MXene-bound O-containing groups. From the Li—O/Li—OH peak ratiosbefore sputtering, the amount of Li⁺ ions is estimated to be 0.3 molesper mole of Ti₃C₂. The LiCl/LiF peak, at a BE of 56.1 eV, is due to theresidues of etching, where some LiF and/or LiCl salts were noteffectively washed away.

After sputtering, the two peaks corresponding to Li—O/Li—OH and LiCl/LiFwere replaced by one at a binding energy of 55.8 eV (which lies in themiddle of those for the two species), with the same FWHM as the peaks ofthose species before sputtering. This might be due to the effect ofsputtering on the Li species.

XPS spectra of the Li is region for Na—Ti₃C₂T_(x) before sputtering(FIGS. 19Aiii) show no sign of Li—O/Li—OH species. Only a peak forLiF/LiCl species was present. This holds true after sputtering as well.The lack of a peak corresponding to the intercalated Li⁺ suggested acomplete exchange between Li⁺ and Na⁺ ions. FIG. 19Bi and FIG. 19Biishow XPS spectra for the Na is region for Na—Ti₃C₂T_(x), before andafter sputtering, respectively. This region was fitted by four species,one each to NaF/NaCl and NaOH/Na₂O. The former was possibly due to theincomplete washing of NaCl salts and the formation of NaF. The lattermost probably originates from intercalated Na⁺ ions. The other twospecies originate from the Ti LMM Auger lines and correspond to the Ti—Cspecies in MXene and TiO₂ (surface oxide). It is worth noting that theBE of all these species shift to a slightly higher BE (about 0.1 to 0.2eV) after sputtering. Again, this might be an effect of the sputteringprocess. The amount of Na⁺ intercalant was estimated from the XPSspectra to be 0.24 moles (before sputtering). Rb—Ti₃C₂T_(x) is similar;the disappearance of the Li⁺ species in the Li is region (FIG. 19Aiv)was associated with the appearance of a species in the Rb 3d region(FIG. 19C). The amount of Rb species intercalated per mole of Ti₃C₂T_(x)estimated from the XPS spectra was 0.16 moles (before sputtering).

Based on the overall results, it is possible to propose a formula forthese MXenes: A^(n+) _(z/n)(H₂O)_(y)[Ti₃C₂T_(x)]^(z−), where A^(n+) is acation intercalant. It is clear that water can be reversiblyintercalated according to the topotactic reaction:A^(n+) _(z/n)(H₂O)_(y)[Ti₃C₂T_(x)]^(z−)+ζH₂O ⇔A^(n+)_(z/n)(H₂O)_(y+ζ)[Ti₃C₂T_(x)]^(z−)  (1)

The driving force for water (de)intercalation is likely the solvation ofthe cations. The M_(n+1)X_(n)T_(x) layers, on the other hand, remainunchanged as negatively charged matrix elements. The 000 l reflectionsare intense and relatively narrow, strongly suggesting the water/cationcomplexes are highly ordered in the c direction.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description and the examples that follow are intended toillustrate and not limit the scope of the invention. It will beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted without departing from the scope ofthe invention, and further that other aspects, advantages andmodifications will be apparent to those skilled in the art to which theinvention pertains. In addition to the embodiments described herein, thepresent invention contemplates and claims those inventions resultingfrom the combination of features of the invention cited herein and thoseof the cited prior art references which complement the features of thepresent invention. Similarly, it will be appreciated that any describedmaterial, feature, or article may be used in combination with any othermaterial, feature, or article, and such combinations are consideredwithin the scope of this invention.

The disclosures of each patent, patent application, and publicationcited or described in this document are hereby incorporated herein byreference, each in its entirety, for all purposes.

What is claimed:
 1. A method comprising, (a) applying a MXene dispersiononto a substrate surface, said MXene dispersion comprising at least onetype of MXene platelets dispersed in a solvent; and (b) removing atleast a portion of solvent so as to provide a coated film of at leastone layer of MXene platelets oriented to be essentially coplanar withthe substrate surface, said MXene platelets comprising aM_(n+1)X_(n)(T_(s)) composition having at least one layer, each layerhaving a first and second surface, each layer comprising a substantiallytwo-dimensional array of crystal cells, each crystal cell having anempirical formula of M_(n+1)X_(n), such that each X is positioned withinan octahedral array of M, wherein M is at least one Group 3, 4, 5, 6, or7, wherein each X is C, N, or a combination thereof and n=1, 2, or 3;wherein at least one of said surfaces of the layers has surfaceterminations, T_(s), independently comprising alkoxide, alkyl,carboxylate, halide, hydroxide, hydride, oxide, sub- oxide, nitride,sub-nitride, sulfide, sulfonate, thiol, or a combination thereof; saidcoated film being electrically conductive and exhibiting: (i) aresistivity in a range of from about 0.01 to about 1000micro-ohm-meters, (ii) an ability to transmit at least about 50%, atleast about 55%, at least about 60%, at least about 65%, at least about70%, at least about 75%, at least about 80%, at least about 85%, atleast about 90%, or at least about 95% of incident light of at least onewavelength in a range of from about 300 nm to about 2000 nm (iii) aratio of DC conductivity, measured in Siemens/meter, to lightabsorbance, (including visible light absorbance), measured as a decadicabsorbance per meter, of at least 0.1 Siemens measured at at least onewavelength in the range of 300 to 2500 nm; (iv) a value of the realdielectric permittivity less than negative one for wavelengths greaterthan a threshold wavelength, for example, 500 nm; or (v) a combinationof any two or more of (i), (ii), (iii), and (iv), wherein theM_(n−1)X_(n)(T_(s)) composition comprises a plurality ofM_(n+1)X_(n)(T_(s)) platelets having at least one mean lateral dimensionin a range of from about 0.1 micron to about 50 microns.
 2. The methodof claim 1 wherein the MXene dispersion is applied dropwise onto anoptionally rotating substrate surface, during or after which theoptionally substrate surface is made to rotate at a rate in a range offrom about 300 rpm (rotations per minute) to about 5000 rpm.
 3. Themethod of claim 1, wherein the MXene dispersion is an aqueous dispersionoptionally comprising one or more surfactants.
 4. The method of claim 1,wherein the MXene dispersion comprising an organic solvent.
 5. Themethod of claim 1, wherein the substrate is rigid.
 6. The method ofclaim 1, wherein the substrate is flexible.
 7. The method of claim 1,wherein the film has surface electrical resistivity in a range of fromabout 1 micro-ohm-meters to about 10 micro-ohm-meters, from about 10micro-ohm-meters to about 100 micro-ohm-meters, from about 100micro-ohm-meters to about 1000 micro-ohm-meters, from about 1000micro-ohm-meters to about 10,000 micro-ohm-meters, or any combination oftwo or more of these ranges.
 8. The method of claim 1, wherein M is atleast one Group 4, 5, 6, or 7 metal.
 9. The method of claim 1, wherein Mis at least one of Hf, Cr, Mn, Mo, Nb, Sc, Ta, Ti, V, W, or Zr.
 10. Themethod of claim 1, wherein M is Ti, and n is 1 or
 2. 11. The method ofclaim 1, wherein M_(n+1)X_(n) comprises Sc₂C, S_(c2)N, Ti₂C, Ti₂N, V₂C,V₂N, Cr₂C, Cr₂N, Zr₂C, Zr₂N, Nb₂C, Nb₂N, Hf₂C, Hf₂N, Ta₂C, Mo₂C, Ti₃C₂,Ti₃N₂, V₃C₂, Ta₃C₂, Ta₃N₂, Mo₃C₂, (Cr_(2/3)Ti_(1/2))₃C₂, Ti₄C₃, Ti₄N₃,V₄C₃, V₄N₃, Ta₄C₃, Ta₄N₃, Nb₄C₃, or a combination thereof.
 12. Themethod of claim 1, the crystal cells having an empirical formula Ti₃C₂or Ti₂C and wherein at least one of said surfaces of each layer iscoated with surface terminations, T_(s), comprising alkoxide, fluoride,hydroxide, oxide, sub-oxide, sulfonate, or a combination thereof. 13.The method of claim 1, wherein the M_(n−1)X_(n)(T_(s)) orM′₂M″_(m)X_(m+1) composition is formed by removing at least 90% the Aatoms from a MAX-phase composition having an empirical formula ofM_(n+1)AX_(n) or M′₂M″_(m)AX_(m+1), respectively; wherein M is at leastone Group 3, 4, 5, 6, or 7 metal, wherein A is an A-group element; eachX is C, N, or a combination thereof; and n=1, 2, or
 3. 14. The method ofclaim 13, wherein removing the A atoms is done in aqueous media.
 15. Themethod of claim 13, wherein M is at least one of Hf, Cr, Mn, Mo, Nb, Sc,Ta, Ti, V, W, or Zr.
 16. The method of claim 13, wherein A is at leastone of Al, As, Ga, Ge, In, P, Pb, S, or Sn.
 17. The method of claim 13,wherein the A atoms are removed by a process comprising a treatment witha fluorine-containing acid.
 18. The method of claim 17, wherein thefluorine-containing acid is aqueous hydrofluoric acid.
 19. The method ofclaim 17, wherein the fluorine-containing acid comprises: (a) aqueousammonium hydrogen fluoride (NH₄F.HF); (b) an alkali metal bifluoridesalt (i.e., QHF₂, where Q is Li, Na, or K), or a combination thereof; or(c) at least one fluoride salt in the presence of at least one mineralacid that is stronger than HF; or (d) a combination of two or more of(a)-(c).
 20. The method of claim 19, wherein the fluorine-containingacid is derived from lithium fluoride and an aqueous mineral acid thatis stronger than HF.
 21. A method comprising, (a) applying a MXenedispersion onto a substrate surface, said MXene dispersion comprising atleast one type of MXene platelets dispersed in a solvent and (b)removing at least a portion of solvent so as to provide a coated film ofat least one layer of MXene platelets oriented to be essentiallycoplanar with the substrate surface, said MXene platelets comprising aM_(n+1)X_(n)(T_(s)) composition having at least one layer, each layerhaving a first and second surface, each layer comprising a substantiallytwo-dimensional array of crystal cells each crystal cell having anempirical formula of M_(n+1)X_(n), such that each X is positioned withinan octahedral array of M, wherein M is at least one Group 3, 4, 5, 6, or7, wherein each X is C, N, or a combination thereof and p1 n=1, 2, or 3,wherein at least one of said surfaces of the layers has surfaceterminations, T_(s), independently comprising alkoxide, alkyl,carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride,sub-nitride, sulfide, sulfonate, thiol, or a combination thereof; saidcoated film being electrically conductive and exhibiting: (i) aresistivity in a range of from about 0.01 to about 1000micro-ohm-meters, (ii) an ability to transmit at least about 50%, atleast about 55%, at least about 60%, at least about 65%, at least about70%, at least about 75%, at least about 80%, at least about 85%, atleast about 90%, or at least about 95% of incident light of at least onewavelength in a range of from about 300 nm to about 2000 nm (iii) aratio of DC conductivity, measured in Siemens/meter, to lightabsorbance, (including visible light absorbance), measured as a decadicabsorbance per meter, of at least 0.1 Siemens measured at at least onewavelength in the range of 300 to 2500 nm; (iv) a value of the realdielectric permittivity less than negative one for wavelengths greaterthan a threshold wavelength, for example, 500 nm; or (v) a combinationof any two or more of (i), (ii), (iii), and (iv), further wherein Mcomprises at least two Group 4, 5, 6, or 7 metals, and theM_(n+1)X_(n)(T_(s)) composition is represented by a formulaM′₂M″_(m)X_(m+1)(T_(s)).
 22. The method of claim 21, wherein M′comprises Ti, V, Cr, or Mo.
 23. The method of claim 21, wherein M″comprises Ti, V, Nb, or Ta, and M′ is different than M″.
 24. The methodof claim 21, wherein M′₂M″_(m)X_(m+1), comprises Mo₂TiC₂, Mo₂VC₂,Mo₂TaC₂, Mo₂NbC₂, Mo₂Ti₂C₃, Cr₂TiC₂, Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂,Ti₂TaC₂, V₂TaC₂, V₂TiC₂, or a combination thereof.
 25. The method ofclaim 21, wherein M′₂M″_(m)X_(m+1), comprises Mo₂TiC₂, Mo₂VC₂, Mo₂TaC₂,Mo₂NbC₂, Cr₂VC₂, Cr₂TaC₂, Cr₂NbC₂, Ti₂NbC₂, Ti₂TaC₂, V₂TaC₂, V₂TiC₂, ora combination thereof.
 26. The method of claim 21, whereinM′₂M″_(m)X_(m+1), comprises Mo₂Ti₂C₃, Mo₂V₂C₃, Mo₂Nb₂C₃, Mo₂Ta₂C₃,Cr₂Ti₂C₃, Cr2V₂C₃, Cr₂Nb₂C₃, Cr₂Ta₂C₃, Nb₂Ta₂C₃, Ti₂Nb₂C₃, Ti₂Ta₂C₃,V₂Ta₂C₃, V₂Nb₂C₃, V₂Ti₂C₃, or a combination thereof.
 27. The method ofclaim 21, wherein M′₂M″_(m)X_(m+1), comprises Nb₂VC₂, Ta₂TiC₂, Ta₂VC₂,Nb₂TiC₂ or a combination thereof.
 28. The method of claim 21, whereinthe M′₂M″_(m)X_(m+1) is in a disordered state.